This guide is not a guide on how to play the game or the best tactics or anything. This is simply a listing of all the practical information I could find within the game on the things available to the player.

In this guide, I talk about what the various stats mean for a character, what classes are available (and what they can do), and what differentiates squads. I have also compiled all of the info I could for all of the items available to the player, excluding three categories:
1) The number of stars for unlocking a weapon and the cost of purchasing the weapon.
2) Purely cosmetic items such as uniforms and facial items
3) Items that are not equipped to soldiers (defibrillators, UAVs, lock picks, and breaching charges)

This is the first guide I've ever written, so if some formatting is difficult to read or some information seems to be missing, please let me know.

Every one of the friendly units in the game has six stats; Reaction, Accuracy, Speed, Health, Evasion, and one special stat that is determined by what class they are. These stats start at 45/100, with the chosen squad providing a bonus to one or two of the stats. Additionally, you get five points for each of 30 levels per character (160 points total) to improve these stats.

Reaction is how quickly the character can acquire and shoot at targets. Being able to get off the first shot can sometimes be the line between life and death.

Accuracy is how often your shots will hit. While enemies in the open are generally easy to hit by anyone, distant enemies or those in cover will require higher accuracy to reliably hit them. This stat can be altered by weapons and attachments.

Speed is how fast your character moves. The higher this is, the more ground you can cover per turn. Armor and helmets will generally slow you down in exchange for more health.

Health is how many shots your character can take. Armor and helmets will usually give additional health at the cost of speed.

Evasion is how likely you are to be missed. I have no clue how this interacts with enemies and accuracy, but the higher it is, the less likely you are to be hit.

Specialization is dependent on the class. Each class has some "ability" that you can upgrade, such as Weapons Sergeants being able to do increased damage or Intelligence agents getting 360 degree vision.

When you make a new character, either while creating a new squad or replacing a dead squad member, there are six classes in the game to choose from. Each one is distinguished from the others by having a unique stat, as well as a seven perks, including two special abilities at levels 1 and 30, with the first of these unlocked automatically at level 1 when you create the character.
The classes are as follows:

The Fire Team Leader is a class focused around improving the team.
Leadership
Leadership provides a stat bonus to any allies within proximity.
Level 01: Draw Fire
If possible, enemies will focus on this unit instead of allies. Can be used indefinitely.
Level 05: Close Knit
This unit has it's stats boosted when in proximity to allies.
Level 10: Assisting Fire
Any ally targeting the same enemy as this Unit receives a 10% accuracy boost.
Level 15: Intimidation
Enemies take longer to acquire this unit.
Level 25: Draw Fire Pro
When using Draw Fire, this unit gains some damage resistance.
Level 30: Morale Boost
Once per mission, this unit can give all allies a large morale boost.

The Weapons Sargent is a class focused on dealing as much damage.
Proficiency
Proficiency increases the damage that this unit deals.
Level 01: Suppression
For a narrow cone of fire in front of this unit, any enemies will run for cover and will not move if they are already behind cover. Any enemies under the effect of suppression will also not fire at anyone. Has a 1 turn cooldown.
Level 05: Suppressing Damage
Suppression will now also deal damage in addition to it's standard effects. Like normal shooting, enemies behind cover will be hit less.
Level 10: Stronghold
Stronghold provides an accuracy bonus if this unit behind cover.
Level 15: Suppression Experience
There is no longer a cooldown on suppression, it can be used indefinitely.
Level 25: Cover Buster
Cover Buster allows this unit to deal damage even to enemies behind cover, especially effective for enemies in half cover.
Level 30: Surprise!
Surprise! lets this unit target enemies even through cover or walls that would normally obscure line of sight. Usable once per mission.

The Medic is a class focused on healing and increased suriviability.
First Aid
First Aid increases the effectiveness of Medpacks.
Level 01: Stabilize
Stabilize allows this unit to use medpacks to revive fallen allies. Uses a medpack.
Level 05: Medpack Plus
Medkits now target and heal all allies within range instead of just one.
Level 10: Innocuous
Enemies are less likely to target this unit.
Level 15: Survivor
This unit's evasion and defense are boosted when health is critical.
Level 25: Stabilize Pro
This unit can now react to enemy fire while stabilizing.
Level 30: Regeneration
Regeneration allows this unit to return to full health. Usable once per mission.

This class focuses on shooting first and not getting shot.
Intimidation
Intimidation makes enemies take longer to target this unit.
Level 01: Sprint
Sprint increases this unit's speed and evasion, but this unit cannot shoot until they have finished moving.
Level 05: Under Pressure
Every enemy targeting this unit provides a stacking accuracy bonus to this unit.
Level 10: Killstreak
Every time this unit kills an enemy, they gain a stat boost until the end of the mission.
Level 15: Dodge This
Any enemy this unit is targeting has a penalty to evasion.
Level 25: Lone Gunman
Everything but speed and health are boosted when this unit is working alone.
Level 30: First Shot
First Shot allows this unit to shoot any enemies they can see or encounter this turn before the enemy has a chance to shoot back. Usable once per mission.

This class focuses on seeing and not being seen.
Intuition
Intuition gives a 360 view of enemies within a radius.
Level 01: Door Wedge
Door Wedge allows this unit to stop an unopened door from being opened by enemies.
Level 05: Lookout
This unit will reveal any enemies within range, even through walls.
Level 10: XP Boost
If this unit survives a mission, the mission's XP payout is larger.
Level 15: Peripheral Vision
+20 degrees to field of view.
Level 25: Sneaky
It takes longer for enemies to detect this unit.
Level 30: Ghost
Ghost allows this unit to move unseen, though this unit cannot fire their weapon. Usable once per mission.

This class focuses on the effective use of grenades.
Explosives
Explosives increases both the throwing range and size of area of effect for grenades
Level 01: Throw And Go
Throw And Go allows the breacher to throw a grenade, and then move with reduced range. Uses a grenade.
Level 05: Frag Pro
Increases Frag Grenade damage.
Level 10: Flasher
Increases Flashbang effectiveness.
Level 15: Hard Shelled
This unit takes less damage from Grenades.
Level 25: Cover Pro
Half cover provides full cover bonuses for this unit.
Level 30: Catch!
Catch! allows this unit to throw two grenades in one turn. Uses two grenades.

When creating your squad, you can choose from one of five organizations from around the world. Each one will provide a unique bonus to one stat for all squadmembers, and will also determine the starting gear they get.

The JTF2, or Joint Task Force 2, is a Canadian special operations team.
The JTF2 provides a bonus to accuracy, and come equipped with Basic Carrier Plates and Mich 2001 helmets. They carry the JTF2 C8A2, which sacrifices some damage compared to the M4A1 in exchange for an accuracy boost.

The Army Rangers are an elite part of the United States Army.
The Rangers provide a bonus health, and come equipped with RBSS vests and Ranger Mich 2001 helmets. They carry the standard M4A1/SOPMOD.

The Army Special Forces are a Special Operations force within the United States Army.
The Army SF provide a bonus to speed, and come equipped with Spear Vests and Special Forces Mich 2000 helmets. They carry the standard M4A1/SOPMOD.

The Navy Seals, or the Sea, Air, and Land Teams, are the special operations group for the United States Navy.
The Seals provide a bonus to reaction, and come equipped with CIRAS vests and Mich 2000 helmets. They carry the standard M4A1/SOPMOD.

The SAS, or Special Air Services, are a British special forces group.
The SAS provide a bonus to Evasion and come equipped with SAS JPC vests and SAS Mich 2000 helmets. They carry the standard M4A1/SOPMOD.

The KSK, or Kommando Spezialkrafte (Special Forces Command), are a German special operations force.
The KSK provide a bonus to a unit's special stat, and come equipped with Taktik Gen III vests and APH helmets. They carry the standard M4A1/SOPMOD.

The Spetsnaz is a Russian military special operations team.
The Spetsnaz provide smaller bonuses to both Health and Reaction, come equipped with Gladiator vests and Kiver-M helmets. They carry the standard M4A1/SOPMOD

Once a squad has been created and the classes chosen, you can gear up your characters. The gear for each character is as follows:

Weapon- Their weapon, which determines their damage, as well as giving accuracy bonuses or penalties.
Attachments- A weapon can have up to seven attachment slots, modifying it's performance.
Helmet- Can give very small bonuses or penalties to health and movement speed.
Armor- Like a helmet, but gives larger bonuses/penalties to movement and health.
Uniform- Entirely cosmetic, this is what they wear underneath the armor.
Face- Entirely Cosmetic, this is face paint or facial accessories.
Equipment- Two slots for grenades, medpacks, or... energy drinks?

Armors will generally provide a bonus to the unit's health at the cost of some speed.

Like armor, helmets provide health at the cost of speed.

Every weapon in the game is defined by five stats:
Accuracy: This is added to your characters accuracy stat. For most weapons, this is 0
Noise: How loud the gun is. The louder the gun is, the farther away it will alert enemies.
Damage: How much damage each shot does.
Rate of Fire: How much time is in between each shot. Beware! This is time/shot, not shots/time, so lower numbers are better.
Range How far the gun can shoot. Although enemies further away require more accuracy to shoot, the range is a hard cap beyond which enemies can't even be targeted.

In addition to these stats, I will be providing the weapon's effective DPS, which attachment slots it has, and what the best individual stats it can have, if that stat can be altered (Note: These are on a per-stat basis. These are not all at once, and, for example, no weapon can have both it's maximum damage and minimum noise at the same time).

These weapons can deal some of the greatest damage, but are balanced by a lack of range.

An extremely short-ranged but deadly shotgun. The shortest ranged and slowest firing of all three, but deals the most damage per shot in the entire game.
Slots: Optics, Gas, and Trigger (No IR, Grip, Light, or Suppressor)

The middle shotgun, it has exactly one tile more of range and a fairly inconsequential increase in firing speed in exchange for reduced damage.
Slots: Optic, Gas, and Trigger (No IR, Grip, Light, or Suppressor)

The longest ranged shotgun, it deals the least damage but with the fastest rate of fire.
Slots: Optic, Gas, and Trigger (No IR, Grip, Light, or Suppressor)

Though the PDW only has half the range of your standard assault rifle, this will hardly ever be noticeable with the maps in the game, making the PDW an absolute damage beast. However, beware the poor accuracy, as even half cover can all but negate this weapon.
Slots: Optic, Light, and Gas (No IR, Grip, Suppressor, or Trigger)

This section represents the bulk of weapons, which are mostly just your standard Assault Rifle fare.

This is the starting weapon for most squads in the game.
Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

Slots: All (Optics, IR, Grip, Light, Suppressor, Gas, Trigger)

The Mk18 Mod0 offers the second-best DPS in the entire game, without destroying all semblances of accuracy like the PDW.
Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

Slots: Optic, IR, Grip, Light, Gas, and Trigger (No Suppressor)

Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

Slots: Optic, IR, Grip, Light, Gas, and Trigger (No Suppressor)

Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

The JTF2 C8A2 is the starting weapon for JTF2 squads. Both the JTF2 and normal C8A2 perform the exact same.
Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

Don't use this. Poor accuracy and damage, and the rate of fire just gives it decent DPS with no chance to improve it through attachments. For a low-damage high-DPS weapon, look to the PDW once unlocked.
Slots: Optic, IR, Grip, Light, and Suppressor (No Gas or Trigger)

The CTAR-21 is capable of being the most accurate gun in the game. Never again worry about missing your enemy with +60 base accuracy, or a whopping +99 accuracy with proper attachments.
Slots: Optic, IR, Light, Suppressor, Gas, and Trigger (No Suppressor)

Notable for being the only weapon that can have attachments fitted to make no noise.
Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

These weapons have a greater than average range.

The longest range weapon in the game. Unfortunately, it also has the worst rate of fire, meaning that it's only useful for attacking enemies who are un-alerted or outside of standard rifle range.
Slots: IR and Trigger (No Optic, Grip, Light, Suppressor, or Gas)

Slots: Optic, IR, Light, Suppressor, and Trigger (No Grip or Gas)

Guns may have up to 7 attachment slots - a Sight, an IR Laser, a Grip, a Flashlight, a Suppressor, Gas, and a Trigger. Each of these attachments have at least 3 options, though only Gas and Triggers are anything other than a straight upgrade, though suppressors are not always advantageous. The attachments are as follows:

Nothing but an accuracy bonus. Buy the highest you can afford.

Also just an accuracy bonus.

Again, just an accuracy bonus.

Hint: These improve accuracy.

These decrease the noise of the gun, but in exchange decrease the damage and accuracy.

These change the gun's accuracy, noise, damage, and rate of fire.

These alter the accuracy, noise, and rate of fire of the gun.

Each unit has two slots in their "Gear" tab for equipment - pretty much grenades and medpacks.

The Frag Grenade is a grenade that will deal damage to nearby enemies. Any enemies in the area will immediately flee and head for cover, so the grenade is best used by breachers or in small rooms if you're aiming to kill people, but also can be useful for driving enemies out of cover or punishing those who stand still.

The Flashbang is the other type of grenade, except rather than dealing damage it causes enemies to be dazed. The further away an enemy is, the less effective the stun is, but even non-breachers can flashbang whole rooms at once. Again, enemies will flee at the sight of one of these flying through the air, but stubborn enemies behind cover will require a bit more effort since the flashbang deals no damage.

The Small Medkit and Large Medkit both are single-use heals. These can be used to heal their holder, or to heal some other ally within range (or to heal all allies in range with a Medic perk). The only difference between the two is that the large medkit returns more health, but also costs more if used.

The Energy Bar/Drink/Stick/Rectangle/thing Increases the character's speed stat.

If you have the Frozen Synapse DLC, it adds one new squad and two new weapons, in addition to a new set of maps.

Petrov's Shard is a set of allied vatforms controlled by the player shapeform from Frozen Synapse.
Petrov's Shard provides a small bonus to accuracy and reaction, and comes equipped without any vests and baclava helmets. They carry Frozen Synapse Rifles.
The Petrov's Shard squad is unique in that any and all equipment other than their weapon is re-textured to solid green. Helmets and vests will be modeled as usual, but different uniforms or face paints do nothing because everything is solid green.

A re-skinned copy of the AR-15, the Frozen Synapse Rifle also has special bullet effects.
Slots: All (Optic, IR, Grip, Light, Suppressor, Gas, Trigger)

A re-skinned copy of the Breacher Shotgun, the Frozen Synapse Shotgun also has special bullet effects.
Slots: Optics, Gas, and Trigger (No IR, Grip, Light, or Suppressor)