What are the different classes in Team Fortress 2

There are nine playable classes in Team Fortress 2, and are split up into three different groups; offensive, defensive and support.
Included in the offensive group is the Scout, Soldier and Pyro. The Demoman, Heavy and Engineer are the defensive classes. Finally, the Medic, Sniper and Spy are from the support group.
Here is some more information about TF2's classes from the official Team Fortress 2 Wiki: