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, choosing a class is usually constrained by the choice of race. As a character progresses, they can specialize their skills by assigning class talent points in one of three trees available for each class.
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.
But, 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.
List of classes in World of Warcraft:
As of the Wrath of the Lich King expansion, the idea of hero classes was introduced. They currently are distinguished in-game from normal classes by requiring a player has played a normal class on their account to at least level 55. Hero classes start at level 55 so far and begin completely differently than normal classes. They share a more common back story than normal classes and have no race restriction (currently). They also begin with a full set of gear and operate for the first few levels in an instanced area separated from other players of different classes.
List of hero classes in World of Warcraft: