A class is the primary adventuring style of a player character which determines the type of weapons and armor it can use, as well as what abilities, powers, skills, and spells it will gain throughout its adventures.
Be aware that the choice of which class to play is constrained by the choice of race; each race has a different group of available classes to choose from. Some classes are currently only available to one race within a given faction. While 3rd expansion will add many additional race/class combinations, many classes will be options for only a few races on a given faction's side.
Each class has three unique talent trees. As a character progresses, they can specialize their skills by assigning class talent points in any of the trees for their class. There are three talent trees for each class. For example, a druid can place their talent points in Restoration, Feral, or Balance.
Collecting class sets is a way players can improve a character when they have reached the level cap. Most dungeon and raid sets are made up of armor items designed specifically for the class; however, faction rewards sets also include trinkets and neck pieces. When players collect more pieces of the set, they can receive set bonuses.
Always remember that it doesn't matter what class you chose as long as you are willing to stand by it and don't give up. If your class gets nerfed (or made less powerful by The Group) always remember that nerfs and buffs are like a pendulum, sooner or later the pendulum is going to swing to your advantage.