In Final Fantasy XI there's a jobsystem. When you create a new character you have to pick a job. If you don't like the job you can always switch to another jobs. When you have completed special quests other jobs will become available.

There's a Support Job too. To get this you have to complete a certain quest. Also you need to have put some time on leveling both jobs. When your main job is level 20 and you select a support job it level will be restricted to 50% of 20, so it will be level 10. A plus is that you can use the abilities of the support job, but you can't level it. You need to have it as main job then.

Here's a list of standard jobs, which will be available at the beginning of the game and the extra jobs, which can select after you have completed a special quest.

¤ Standard Jobs

Warrior
Specializing in battle and fighting, the warrior acts as a shield and tank, keeping enemies away from less powerful friends and hacking down enemies with their b attack.

Monk
The Monk disdains weapons in favor of his own bodily abilities, the monk can attack his enemies with explosive attacks and deal out punishment with his own fists.

White Mage
The White mage utilizes white magics to heal and empower allies.

Red Mage
The powerful red mage can use can use both black and white magics, as well as using his sword to deadly effect.

Black Mage
The black mage specializes in devastating black magic, to bring down his enemies before they get close.

Thief
The Thief specializes in obtaining items, especially other peoples. Not as b in frontal assault, the thief excels at attacking from behind.

¤ Extra Jobs

Paladin
Paladins specialize in close-range combat, just like warriors. However, through long hours of spiritual training, paladins have acquired the ability to cast white magic spells, making them even more valuable on the battlefield.

Dark Knight
To strengthen themselves in battle, not only have these outlaw warriors mastered the ways of numerous weapons, but they have also acquired certain black magic spells to torment their enemies.

Ranger
With unparalleled tracking abilities and skill with the bow and arrow, rangers are experts in the field of hunting.

Samurai
Through the study of Far Eastern martial arts techniques, these lone warriors have learned to devote their lives to their cause and to their blades, the mighty katana.

Dragoon
With their lances in hand and their faithful wyverns by their sides, dragoons suprise their enemies with their extraordinary jumping abilities.

Bard
A bright tune to soothe a party's wounds... A brisk march to raise a party's spirits... A forbidden chant to weaken a party's foes... The bards of Vana'diel use music as their weapon.

Beastmaster
These experts in monster research have trained tirelessly to communicate with--and even control--the animals of Vana'diel

Summoner
By making pacts with the the legendary "avitars," these heretic mages have unlocked the secrets to the forbidden magic of Vana'diel.

Ninja
Stict training in the forbidden arts of the Far East have transformed these fighters into cold, hard killing machines.

Blue Mage
Employing the legendary arts of Aht Urhgan, these formidable fighter-mages employ elegantly curved blades for close combat, while decimating their enemies from afar with fell magic mastered from their opponents.

Corsair
Descendants of the dauntless pirates that once scourged the seas of the Near East, corsairs rely on the "Hexagun", a multi-barreled revolver, and elaborate luck-based abilities to alter the stakes of battle.

Puppetmaster
These wandering performers entertain crowds and confound their enemies with a customizable puppet known as an "automaton." Puppetmasters can also fall back on hand-to-hand fighting skills that utilize the flexible movements of their art.