This is not an exhaustive list, each job has tremendous depth, and subjobs can be an artform in and of itself. So read it with a grain of salt.Remember, you do NOT have to reroll to try another job. You can switch jobs as often as you want without penalty, all progress you make will be waiting for you if you switch to another job and then decide to come back to one.
Basic jobs- These jobs are available to everyone at the start of the game.
WAR- Warrior. This is the basic tank job at lower levels, turns into a damage dealing job. They get a special ability called "Provoke" that gets a monsters attention and keeps it. WAR is a popular melee sub, escpeidally for tank jobs because of this ability.
MNK- Monk. Hand to hand combat, this is a pure damage dealing job. They smack things, they smack things hard. This is argualbly the easiest job to play when starting out, not much to do or think about. At 18, MNK typically subs WAR for backup Provoke.
WHM- White Mage. This is the main healing class. Hard to solo, can;t take alot of damage. Buffs the party against all sorts of attacks, physical, magical, elemental. WHM can mass cure a party starting in the teens. It is the parties job to keep the WHM alive so the WHM can keep the party alive. WHM is also often a subjob for soloing, or for other mage classes who can main heal. WHM subbing NIN can be very cool solo, though, if you insist on soloing.
BLM- Black Mage. BLM nukes. Period. At higher levels they can do some debuffing and enfeebling, but BLM is good at blowing stuff up and closing skill chains with Magic bursts for extra damage. BLM is really hard to solo, but not quite as hard as WHM. Nuke nuke nuke. BLM subs WHM for backup curing and extra MP.
RDM- Red Mage. RDM uses a sword, can use both white and black magic. RDM can melee, main heal, enfeeb or deal damage. They specialze in enfeebling the mobs, and at level 41 get an ability to cast a spell called refresh, whcih lets party members recover MP without resting. A RDM and a BRD in a party usually means nonstop killing. You never have to rest. RDM can sub BLM (for extra MP) or WHM (to act as Main healer). RDM/NIN make excellent soloers at higher levels. I play a RDM as my main job.
THF- Thief. THF is the party puller. THF can sub WAR (for backup provoke), NIN (to take less damage) or RNG (for hunting mobs and extra Ranged attack skill). They aslo get the ability to steal items from the mobs, and in the teens, get Sneak attack, which lets them do a WHOLE lot more damage than they normally can so long as they attack from behind the mob.
Advanced jobs- These can be unlocked once you have a basic job levelled to 30. They each have quests associated with them, of varying difficulty. Some require fighting to unluck, others travelling.
PLD- Paladin. These are tanks, pure and simple. They maintain hate mostly with a WAR sub and provoke, massive self healing, and at higher levels, spells like flash. They use a sword and shield, sometimes a great sword, and some use a staff at higher levels (51+)
DRK - Dark Knight. This job subs WAR for back up Provoke. They usually carry a scythe or a great sword. They use Black Magic for extra damage and debuffing. These are party damage dealers, they have big slow weapons, when they hit, they do alot of damage. DRK will sometimes be the party puller.
RNG- Ranger. This is the ranged attack job. Bows, crossbows, and guns. They do termendous damage with their weapons. RNG will often sub THF or NIN. These are almost always the party puller. Arrows can be very expensive. You can solo RNG reasonably well, I solod to 24, and plan on soloing it to 30. RNG also gets widescan, which lets them find mobs and NPC's on the map.
BRD- Bard. This job uses a special type of magic (Singing magic, duh) and play instruments to enhance that magic. They do not use MP for these spells. They pretty exclusively sub WHM to act as a backup healer in a party. BRD makes EVERYTHING better, they affect the entire party with their spells, either with accuracy bonuses, attack bonuses, health, or MP regen, and can debuff the mob with elemental weaknesses and various DoT effects. BRD can be the party puller.
BST- Beastmaster. A pet based job. Beastmasters fight by charming animals in the wild and using them to fight. This is typically a solo job and subs WHM. BST is handy in a BCNM 20. If a BST is in your party, he/she is usually DD. BST's get less experience when using pets. BST gets a lesser ranged widescan to help locating suitable pets.
DRG - Dragoon. Who DOESN'T love dragons? Another pet job. The dragon follows you around always outside (unlike BST pets, who will eventually turn on you). These guys use spears, and are typically damage dealers. The Wyvern doesn't cause an experience penalty, and basically acts like another member of hte party. This job is a damage dealer and frequently gets asked to pull. Common sub for this in a party is WAR for the backup voke, though subbing mage makes your Wyvern do cool things.
SMN- Summoner. SMN us MP, and have alot of base MP. They do what you think they might, summon things. These Avatars are based on elements, and are extremely cool to watch. They get access to Tier II magic via their avatars early. SMN almost exclusievly sub WHM in parties, and get asked to main heal. They happen to make excellent Damage Dealers with their avatars. I solo'd SMN to 30.
NIN- Ninja. NIN are main tanks after level 12. They use special tools to cast special types of magic (not MP based). At level 12, they get an ability to cast "shadows" which absorb attacks from mobs. The shadows are like the white magic "Blink" They also dual wierld weapons after level 10. NIN usually sub WAR (so they can Provoke for tanking). Ninja tools are very expensive.
SAM- Samurai. Damage dealers, sometimes pullers (see a trend yet?). SAM is master of hte weapon skills. They sub WAR or THF. Alot sub THF for the "sneak attack" ability to add extra damage to the already amazing damage they do.
ToA Advanced Jobs- These are additional advanced jobs. people are still figuring these out.
BLU- Blue Mage. BLU gets abilities by fighting monsters and learning them from them when they are used. I have seen BLU be front line, almost tanks, pullers, and back line support casters. It was easy to solo this the few levels I have tried, you can deal tremendous damage in a short amount fo time and heal yourself.
COR- Corsair. Pirates. They work alot like BRD, so you see people trying to play them like BRD. They use "magic" dice to generate their abilities and effects on the party. They get an ability early on that increases the amount of experience gained by the entire party. They can use guns. You will see COR/WHM, COR/NIN and COR/RNG.
PUP- Puppetmaster. THe last of the pet jobs, PUP gets an automaton that stays with you at all times (like the DRG Wyvern), and you can customize it as your level with various attachments. PUP themselves use Hand to Hand, I have not seen a PUP in a party yet. When I levelled the few levels I had, I subbed MNK or WHM.
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