Choosing your class in World of Warcraft is a big decision.  Once you choose it, the only way to change classes is to start a whole other character.   Below I have provided some information that will hopefully give you what you need to make the choice that best suits you and what you want out of your game experience.

Druid

Druids are versatile hybrids with various shapeshift forms in which they can tank like a warrior (Bear Form), stealth and fight in melee like a rogue (Cat Form), heal like a priest (normal or Tree Form, and cast spells like a mage (normal form or Moonkin Form). As such they are the only class that can fill all 4 main roles. However, unlike other hybrids, druids do not perform several roles at once but instead shift between roles. With talent specialization, Druids can excel at any of their playable roles in a group and/or raid.

Hunter

Hunters are the only class that can effectively use ranged weapons (e.g. bows or guns) as a primary source of damage. Very adept at kiting, they have plenty of tools to allow them to avoid melee combat, where they are fairly weak. Hunters may tame many varieties of beasts to serve as combat pets, which along with an array of scouting, tracking, and survival abilities make them excellent solo characters.

Mage

Mages are ranged magic damage dealers with very powerful direct and area effect damage spells, but weak armor and health. They also feature a variety of utility spells, including crowd control, teleportation and group portals to capital cities, and conjuration of food and water. Mages are reliant on mana for almost all of their abilities.

Paladin

A hybrid class focused on defensive melee skills and healing abilities, paladins have very strong group support and buff abilities. They may wear heavy plate armor and carry shields like a warrior, but are also efficient healers, which combine to produce their renowned durability in combat. With talent specialization and equipment selection, paladins can fill the role of tank, offtank, or healer, and excel when called upon to spot-fill all these roles in a group setting. They can even sacrifice themselves to provide wipe recovery when needed.

With the release of the Burning Crusade, Paladins are no longer exclusive to the Alliance. Blood elves (Horde) can now play as Paladins.

Priest

Priests are the quintessential healer, and easily the most versatile class in that role, though they may also specialize to become powerful offensive casters. Priests have a wide variety of healing utility spells, along with a few tricks, helpful and otherwise, such as Power Word: Fortitude, Mind Control and the almost always-useful Dispel Magic. Besides their numerous healing spells, they can choose to pursue Shadowy arts, gaining the ability to assume a temporary Shadowform, increasing damage while sacrificing all but their vampiric healing abilities.

Rogue

Rogues excel at dealing melee damage and incapacitating their foes, particularly low health and low armor targets such as casters. Stealth gives them a tactical advantage and allows them to start a fight on their own terms. Other important abilities include lockpicking and the creation and handling of poisons. Also, a huge power is the Rogue's ability to stun and incapacitate for a large period of time. This, paired with the rogue's high Damage gives him a huge edge over his opponent. Rogues do not use mana for their combat abilities. Instead they use energy and generate combo points which are used to execute finishing moves.

Shaman

Shamans are one of the most versatile classes in the game as they can switch seamlessly from offensive caster to healer to melee DPS as needed, though at later levels they often specialize in one of these roles. Shamans can make good support characters, with the ability to step in and fill any role that may be lacking at the moment. Their high burst damage and healing also make them quite good at PvP. They are able to drop stationary totems on the ground which provide a variety of effects, many of which are useful (although minor) to the entire party. They are unique in that they can provide complete wipe recovery by self-resurrecting and then resurrecting the rest of the party.

With the release of the Burning Crusade, Shamans are no longer exclusive to the Horde. Draenei (Alliance) can now play as Shamans.

Warlock

Warlocks enslave demons and call upon dark magic and curses to dominate and destroy their foes. Like hunters, warlocks employ combat pets, but these are summoned demons rather than tamed beasts. They are primarily ranged magic damage dealers with a focus on damage over time in addition to nukes and AoE, but lack the extreme burst damage capabilities of a mage. Demon pets and fear spells give them a variety of tactical options, and they have several utility spells and abilities, including summoning party members and creating healthstones and soulstones.

Warrior

Warriors are close-combat fighters and usually fill the role of primary tank, as they are able to withstand and inflict fierce physical punishment and have the most versatile means of attracting and disabling mobs, keeping aggro off other characters. They can also contribute very strong melee DPS when needed. Warrior abilities use rage, which is generated by dealing and taking damage, but decays over time while not in combat. Warriors are highly gear dependent, as their ability to deal and take damage is controlled directly by the quality of their weapons and armor.

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