There are 9 classes with unique attributes such as health, run speed, weapons, and other special abilities. Each class has 3 weapons, a Primary, a Secondary, and a Meele weapon (spy and engineer also have a fourth equip-able for a special ability).
General info: Switch weapons with scroll wheel or numbers 1-3. 'q' quickswitches between last weapon. You can walk through your friendly team members. There is no friendly fire, you cannot hurt team members with your weapons. Explosive weapons like rockets and the demoman's weapons can damage only the player shooting them if they explode too close. Explosive projectiles cannot shoot through a friendly sentry gun/buildings, though they will not damage the building.