Entry Fragger - The first one in. - Aggressive. Great shot. Great communication.
Playmaker - Get trade kills if the Entry Fragger goes down. - Aggressive. Great shot. Great in the clutch. Great communication.
Strat Caller - Calls the round strategies (and/or ct positioning). - Great knowledge of the game. Great communication.
Support - Aid those in need of a pop flash or smoke. - Great knowledge of nades. Passive. Can stay calm and pick up trade kills. Great communication.
Awper - The primary awp. - Great reaction time. Great aim. Can entry. Great communication.
Lurker - Unpredictable to keep the enemies on their toes. - Great knowledge of rotations. Great positioning. Great use of sound. Great communication.
Communication is a must for every team at every position. The better your shot, the better you'll be at your respective role (obviously). These "roles" are not official, and I'm sure there are many more that could be described.