Originally Posted by PHB4E
Each character class specializes in one of four basic unctions in combat: control and area offense, defense, healing and support, and focused offense. The roles embodied by these functions are controller, defender, leader, and striker. The classic adventuring party includes one character of each role: wizard, fighter, cleric, and rogue.
Character roles identify which classes can stand in for each other. For example, if you don’t have a cleric in your party, a warlord serves just as well in the leader role.
Roles also serve as handy tools for building adventuring parties. It’s a good idea to cover each role with at least one character. Lest the Gods of D&D teleport to your location and smite thee with a d20 Club of Unconsciousness and defile your body with tattoos of butterflies and "I luv Britney" writing inside of hearts. If you have five or six players in your group, it’s best to double up on defender first, then striker. If you don’t have all the roles covered, that’s okay too—it just means that the characters need to compensate for the missing function.