Before we begin, let’s go over some job mechanics. First of all, jobs have levels separate from your character levels. By performing and successfully executing any action except “Flee!” in battle, you will gain a hidden number of job points (referred to as JP in this guide). As your job levels up, you will reap benefits such as more powerful special abilities and higher hit count. There's no way really to know how many job points you have, but the game will inform you when your job levels up.
If you want to specifically level up your jobs quickly without risking dying (ex. if you're already at level 99) then go to Altair Cave and repeatedly use the "Guard" action on the Goblins inside.
When changing jobs, you will notice that your character has poor performance and that by their icon in the Status screen, there is a grey arrow pointing down. What is this? They are caught in the middle of “transition” . Think of it like the character learning (or re-learning if you’re switching back) the job. It takes a bit of time. The higher the levels of the jobs involved, the shorter the transition time. Keep track of your character's transitions so that you aren't fighting important or hard battles during that period.
Finally before we jump in, I’d like to leave you with some advice. You do not need to reach job level 99 to perform well in combat, or beat the game. A job level of around 20 - 30 is sufficient. If a character never switches jobs, you will find that they naturally tend to have a high job level. But don’t worry if you switch jobs after restoring a Crystal. The Earth Crystal is especially notorious for causing a lot of switches right before the final boss due to many of its jobs being upgrades to previous jobs. Just go ahead and do it, level grind a little bit if you are worried.
Being at max job level unlocks a nice bonus in the form of the Legendary Blacksmith sidequest but otherwise you don’t need to aim for it. (DS players will need the data from a long-gone Wi-Fi event, Steam users will find that it is automatically unlocked when a character hits job 99 and you are sufficiently advanced into the game’s plot.)
All jobs will first have a melee, magic, or hybrid classification based on what they primarily rely on to deal damage. After that, weapon types that can be equipped will be listed, and finally, I will give a brief overview of the job and my opinions and recommendations. Jobs are sorted by the crystal that unlocks them so you have an idea of what you have first, and what you have last.