Question

[Nutz]

You can only equip so many abilities at once (and only certain ones). For instance, you could level up archer, thaumaturge, conjurer, pugilist, and lancer to 30. At that point you could use any of those classes and equip 6 cross class skills (skills learned on another class). They're a bit restrictive though. When you go to equip cross class skills it only shows you the ones you can use.

At that point you can also complete quests to get soul gems (equip to change to a job). With that particular combination of classes you'd be able to change to Monk, Black Mage, and Bard. Jobs share abilities and traits with their base class (PGL -> MNK / THM -> BLM / ARC -> BRD), gain their own abilities through quests, and can assign cross class skills from only 2 other specific classes and only half as many abilities (so 3 at 30 -- 5 at 50).

I suppose it gets a bit complicated, but here are some links to get you sorted:
All job requirements
Skill trees

That second one lets you select a class or job to view all the skills it could use, what class they are learned from, and at what level.