Every job comes with a list of weapons that can be equipped, skills that are enabled for use, and abilities for passive benefits. Jobs also give additional skills and abilities by leveling them up, through Job EXP via battling or consuming seeds. A job is considered "mastered" at level 10 and cannot level further, but most jobs unlock stronger versions once mastered.
Jobs can be freely changed by the priest at Ilias Temple.
Some jobs require a Job Change Item before it can be selected, which are found throughout various locations in the world.
Basic Jobs are the beginner jobs and are mostly for testing out playstyles and roles for certain characters. They level up quite fast because of it.
Intermediate Jobs are for when you know what you want to make a character and wish to get stronger, but they take longer to grow.
Advanced Jobs are unlocked after you complete significant events and will lead your characters to become even stonger. They take even longer to master as expected.