There are quite a few differences between Empire Total War and previous Total War games.

Routing: Many of you are familiar with the term routing, it pretty much means that the unit is fleeing from the battle in panic. In Empire there are two different types of routing. The first one is broken, this is when the unit is routing but has a chance to regroup and come back into the fight. The second stage is called shattered, this is when the unit is so demorilized that there is absolutely no chance of it coming back into the battle. Be wary of those units that are only broken as they can come back later, it is best to chase them down with cavalry until they are shattered.
Another thing, a flashing banner means that the unit is wavering, unlike previous games where a flashing banner meant that the unit had broken and you could move on to the next engagement.

Morale Shocks: In Empire there are many units with the trait that says Resistant to Morale Shock what this means is simply that the old hammer and anvil trick will not cause a unit to route the moment it happens but that the unit will hold it's ground for a longer time than one would expect coming from an older Total War game. (Try to visualize what fighting Urban Cohorts is like and you might get an idea of how it works)

Cavalry charges are no longer completely op: In previous games such as Rome and Medieval 2 many players could expect a heavy cavalry charge to break through an infantry line most of the time (except against spearmen). In Empire a frontal cavalry charge is completely suicidal due to the fact that the cavalry will simply charge into a hail of bullets and any survivors will most likely be met by rows of bayonets. Another thing that one must remember is that line infantry can now form up squares that are deadly to cavalry.

Exhaustion: There has always been exhaustion in Total War games, however in Empire units tend to get tired much more quickly (in my opinion) than in previous titles. As before exhausted units suffer severe penalties in melee and I'm assuming that they also suffer penalties in fire rate and accuracy.

As in all other Total War games the terrain is a vital part of any battle. As before the best place to set up your units is on hill and ridges. Cannons(except for mortars and howitzers) tend to shoot in an almost straight line so placing them above your troops helps to avoid casualties. Be wary of enemy roundshot which tends to bounce leading to a few losses even when behind cover. However shrapnel shot will simply hit the ground or safely pass over the heads of your troops(unless they are on top of a hill where they make tempting targets). As before, units going up a hill tend to have a melee penalty and units going down a hill gain a hill advantage.
Terrain can also help keep your troops safe from musket fire, by placing your troops behind a hill the enemy will be unable to shoot at them directly and will wind up wasting their shots.

Buildings: Certain types of buildings in the battlefield are capable of garrisoning troops on the battlefield. This provides great cover from musket fire while allowing your men to fire back. It also protects troops from cavalry since it is impossible to a cavalry unit to enter a building. However, a cannon can destroy the building and kill off any who are trapped inside when the building crumbles.

Something fun to note, those flimsy little wooden fences can stop a cannon ball, however be careful as canister shot can still go through it.

As with all other Total War games infantry will make up the bulk of your armies and they form the backbone from which the battlefield rotates around. Here's some basic info on units and tactics.

Line Infantry: This is your basic infantry unit in Empire Total War. They have average accuracy and reload rate. They are resistant to morale shocks and have fairly decent attack and defense. They make up the bulk of every army in Empire, late game battles give line infantry more tactics that can be used such as the square formation, rank fire drill, and fire and advance drill.
Edit: Maratha Confederacy line infantry do not learn rank fire drill

Superior Line Infantry/Guards: Much stronger and more accurate versions of Line infantry, they also inspire troops increasing the morale of units that are near them. They also learn the rank fire, and advance drills.

Grenadiers: These are large men who have grenades in their arsenal. These units are only comprised of half the standard number of men for line infantry but they are stronger and more accurate. Late game grenadiers learn the platoon firing drill.

Hand Mortars: These are strange troops. They shoot up to 6 grenades at a much farther distance than grenadiers and have a decent attack, I believe they are weak against cav as they don't carry around bayonets. I believe they learn the rank fire drill though I am not sure.

Light Infantry/Riflemen: These troops have much longer range than other infantry units and have much better accuracy. They are physically weak for the most part but are capable of deploying stakes to ward off cavalry charges. These units know the light infantry doctrine and skirmishing tactics.

Swordsmen: Swordsmen have very high attack and defense stats and can cut through even the best line infantry. They also have fairly good morale. However, they are weak in a headlong charge against line infantry as they are bound to be shot to pieces before making contact. They are also susceptible to cavalry charges as (unlike line infantry whose muskets make a makeshift pike) they have no real way of defending themselves from a charge.

Native Warriors: These warriors are fairly strong in a melee and are relatively cheap enough to where they can be given extra experience adding to their combat prowess. Like swordsmen they have no range and must close into a melee as quickly as possible.

Medicine Men: These troops are very strong in melee, inspire other troops, and can shoot reasonably well. However, they only have 5 shots and these units are comprised of half what a normal unit would be.

Archers: They may seem archaic but archers are actually kind of useful. Archers have the advantage of being able to shoot over your men to get at the enemy. Armenian archers can also place down stakes. All archers can skirmish.

Pikemen: Early game unit, they have pikes but fight with swords unless you put them in a square or pike wall formation. When using these formations they are immobile and are as strong as regular line infantry with bayonets.

Rank Fire Drill: The unit will shoot with the front line first then these will kneel down and reload while the next line shoots and so on until it becomes time for the first rank to shoot again.

Platoon Firing Drill: With this drill the units fire off a round platoon by platoon, basically the guys on the left shoot first and reload as the next 2 or 3 guys in the line shoot until the last person on the right has shot and the process repeats again.

Fire and Advance Drill: The unit moves up while keeping constant fire on the enemy. It is like the rank firing drill except that the units will space themselves out a bit more and will slowly move up after each volley.

Square Formation Drill: The unit will form a square so that it is invulnerable from all sides from a cavalry charge. It is deadly to cavalry that fight it but it is immobile and susceptible to infantry and cannon fire.

Grenades: These little iron balls explode a short while after they are thrown, they cause massive casualties and have a powerful effect on the morale of whoever was within the blast radius.

Line Infantry Doctrine: Light infantry will space themselves out more and will do their best to use the terrain to their advantage so as not to be hit by arty as they close in.

Skirmish: Units with skirmish turned on will avoid melee at all cost. They will fall back as soon as an enemy unit gets too close to them.

Cavalry in Empire has a slightly diminished role you cannot simply have an army of nothing but cav and expect to win. That does not mean that they are useless however, cavalry can be used to chase down skirmishers and kill off artillery. Cavalry are also important for their ability to flank an enemy army and smash into its sides and rear. As in previous games cavalry are best used to charge into the rear of an enemy army. After pulling out your cavalry from a fight try waiting for them to form up properly or else their charge will be weakened. Oh and never charge an infantry unit from the front as your cav will get shot down before they can close in and never NEVER charge them into a unit that is formed up into a square.
Most Cavalry units can learn the wedge and diamond formations.

Light Cavalry: Light cavalry have a decent attack and charge, however they have a very low defense which means that they won't survive long after a charge. In other words you have to pull them out almost as soon as you charge in with them. They are usually very fast cav and some have good stamina which allows them to quickly get around the enemy and still be ready to fight.

Lancers: Lancer cavalry are usually light cavalry that have extremely poor attack and defense. However, they have a powerful charge and are usually fast units. However, they must not get bogged down in a melee as they will always lose. You must only let them charge in and then quickly retreat in order to get ready for another charge.

Missile Cavalry/Dragoons: These cavalry are usually classified as light and most have very poor attack, defense, and charge. However they have a ranged attack and are usually extremely fast with good stamina. They can be used to ride around an infantry unit and shoot at it from behind. They can also be used to harrass enemy cavalry since they can outrun heavy and medium cavalry.

Regiment of Horse: This is the standard cavalry unit for the European nations. They can be seen as medium cavalry who have decent attack, defense, and charge.

Heavy Cavalry: As the name suggests, these are the strongest and best armored cavalry units. They are rather slow when compared to light cavalry but they are immensely powerful and can kill off all other cavalry (if they can catch up to them). They have a powerful charge (not as powerful as lancers) and can stay in a melee against infantry though it is highly advised not to do this. As with all other cavalry, it is best to charge in, retreat, and then charge again until the enemy breaks.

Heavy Lancers: These units are heavily armored lancers that serve the same role as lancers. They have fairly good attack and defense and boast a powerful charge. However, they are at a disadvantage against other heavy cavalry and are best used to crash, retreat, and crash again.

Diamond Formation: An evolution of the wedge formation, this formation sets up your cav in diamond formation that is more flexible than the wedge formation. Both formations increase the unit's charge.

In previous Total War games artillery has played more or less an important role in battles. One must never leave their artillery where it might easily be destroyed and that's about it. Do not place any units in front of any artillery piece that shoots straight as they will be blown apart when you shoot.

Fixed Artillery: Fixed artillery usually refers to mortars and rockets than cannot be moved once they are placed and have a very long range. This type of artillery tends to be incredibly inaccurate and once the enemy is close enough they cannot be targeted anymore. Mortars can use concussive shells and quicklime which (when they hit) can decimate the enemy's army.
It also refers to sakers and other early cannons that cannot be moved and have the same properties of foot artillery.
Another type of fixed artiller are 64 iber guns which are extremely large cannons that are larger versions of sakers. They have a greater range than most artillery (just beneath mortars and rockets) and can shoot roundshot, canister, and scattershot.

Foot Artillery: Foot artillery are cannons that can be moved and place where they are needed. They move slowly but are more accurate than fixed artillery. They have reduced range but can fire roundshot, canister, and scattershot.

Horse Cannon: These are fast and light cannons that can move at around the same speed as heavy cav. They are much weaker cannons but can give you increased options on where to place them. They are also fast enough to quickly take over positions that would normally take a cannon a long time to reach. They fire roundshot, canister, and scattershot.

Howitzers: This type of cannon is able to lob projectiles over obstacles making cover useless. They have a slightly shorter range than horse and foot artillery and move around at the same speed as foot artillery. They fire roundshot, quiklime, and concussive rounds.

Roundshot: The basic type of ammunition used by all cannons, it is a metal ball than is devestating to structures and can cut through several men or horses.

Canister: This is very short range shot that effectively makes your cannons into giant shotguns. They are devestating to any nearby units.

Scattershot: This is a mixture of roundshot and canister and has a slightly shorter range than round shot. It is fired like a roundshot but after a certain distance it opens up and sends shrapnel that cuts units to ribbons.

Quicklime/Carcass shot: This type of ammunition is rather nasty, it shoots like a round shot but it opens up on top of the enemy spraying them with lime causing heavy casualties.

Concussive/Explosive rounds: This type of ammunition explodes when it hits the ground spraying shrapnel in all directions. It can be extremely lethal for any unit that is caught in the blast.