Q: What crafting classes are best for my combat class?
A: Check out this post.
Q: How many crafts can I have at once?
A: All of them. You can level every crafting class to 50 on one character. Plus, like combat jobs, there are cross-class abilities in each crafting class.
Q: Can I make a DoH or DoL class my main?
A: Yes! You can level it all the way to 50. Keep in mind though that story quests are designed for combat classes. This means you might want a DoW or DoM job leveled to 15 or 20, so you can do story quests and get access to airship travel, and join a Grand Company respectively.
Q: Do crafting and gathering classes have special armor?
A: Yes! Crafting and gathering classes have armor with unique attributes like craftsmanship, control, gathering, and perception. If you want to maximize your crafting capability, you'll want to get the best crafting gear you can. You can even get crafting and gathering artifact armor!
Q: What are Classes and Jobs? How are they different?
A: Please see this wiki page.
Q: What crafting class goes with my fighting class?
A: This post breaks down categories of crafted items by crafting class, so you can easily see what crafters make your weapon and armor.
Q: Can I play as any class on a single character?
A: Yes! Once you complete the level 10 class quest for your starting class, you can unlock other classes. Once you unlock a class (which involves a short quest) you can switch to and from that class at almost any time. This includes crafting and gathering classes. Your progress in one class does not carry over to another class - so even if your Gladiator is level 50, your Thaumaturge still starts at level 1!
Q: Should I level multiple classes?
A: Yes! Leveling a class gives you access to cross-class abilities. Every 5 levels, you get one cross-class ability slot. Certain abilities from other classes can be slotted into them, letting you use them regardless of class. For example, after leveling Conjurer a bit, you can take it's Cure or Protect spells and use them on your Lancer.
Some abilities are restricted to certain classes, and some cannot be used outside their native class. /u/hobogoblin has compiled a list of abilities and which classes can use them here.
Q: How do Jobs work?
A: Jobs are a more-specialized version of a combat class. When you level a class to 30 and another to 15 (the second class depends on the job), you can do a quest to unlock the job. You'll be given a stone; whenever it's equipped you become that job (so a Marauder becomes a Warrior). The job's level is always the same as the level of it's class, and xp earned on a job is applied to the class. The job gets special abilities, but in return many cross-class abilities are restricted.
Q: What do stats do?
A: Strength - Increases melee attack power for most jobs and the percentage of damage mitigated by block and parry.
Dexterity - Increases ranged attack power, melee attack power for Rogues and Ninjas, and the chance of blocking or parrying an attack.
Vitality - Increases maximum HP.
Intelligence - Increases attack magic potency.
Mind - Increases healing magic potency.
Piety - Increases maximum MP.
Do bonus (attribute) stat points affect all my classes?
No! Combat classes get bonus points separately. So if you throw all your Lancer's points in Strength, your Archer will get separate points to put wherever you want.
Q: What is the best race for my class?
A: The difference between the best race for a stat and the worst race for a stat is 5 points. You will have hundreds of points in your main stat by the time you reach 50. The difference between each race is quite minor. Plus, a race that's "good" at one job is likely not for another. The same is true for your choice in god.
If you want to min/max for one stat, there is a nice rundown here.
Q: How does my class play?
A: /u/ahfuah has made an excellent thread about the classes that were playable in beta phase three here. This post is somewhat outdated as it does not include arcanist, rogue, or account for game mechanics specific to the jobs (e.g. Ninjas and mudras; Warriors and defiance). It also does not factor in balancing updates implemented to classes since the game's launch.
Q: Are there plans to add new classes or jobs?
A: Yes. Rogue & Ninja was released in 2.4. Dark Knight, Astrologian, and Machinist are jobs coming for the expansion.