Where neither Seraphim nor raindrops go...
The Classes of 4th Edition Dungeons & Dragons can be broken into four roles:
Controllers can be best explained as classes that are designed to have an indirect effect on the battlefield. These are the kind of people who can change the condition of terrain, create distracting influences in the environment, or even help maneuver enemies into a correct position.
Such classes include: Druid, Invoker, Wizard, Psion
Defenders are your typical tank characters: they are designed to take a beating and keep on ticking. Some are designed to take more of a beating than others, while some are designed to give as good as they get, but all are useful when your party gets into the thick of it.
Such classes include: Fighter, Paladin, Warden, Swordmage, Battlemind
Leader classes aren’t necessarily the leader of the group, oddly enough. Playing a Leader class merely means that you have the ability to help your party in ways that other classes can’t; Leaders generally use their powers to create complex tactics with their allies, allowing them move around the battlefield more quickly, be more precise with their attacks, or even heal themselves in the middle of combat.
Such classes include: Bard, Cleric, Warlord, Shaman, Ardent
Strikers are solely designed to deal damage. Whether sneaking around the edge of the combat and diving in when the enemy least expects it, staying in the back and picking off enemies with arrows or spells, or even diving into the middle of a fray and swinging wildly at your foes, Strikers are there to capitalize on the mistakes your enemies will make.
Such classes include: Avenger, Barbarian, Ranger, Rogue, Sorcerer, Warlock, Monk