Are new jobs simply advanced classes or different system?

[Volsung]

Think of classes as broad varieties of skills that stem from leveling using a certain weapon. If you just use the class you can use a lot of skills because you can cross class almost whatever you want.

Think of a job as a niche role to perform as while on a specific class. Also you will be limited on what skills you can cross class.

Here I'll give 2 examples.
If you want to be a gladiator and deal damage, you will use the class and equip many attack buffs from other classes and high damage weaponskills. If you want to be defensive and offensive, you will also use the class and eva/def skills from other classes. But if you want to focus on only tanking, you can opt to turn the pld job on. This will give you special tanking skills that you learned from quests. You might not be able to use attack buffs from other classes though so your DD will go down while your tanking goes up.

If you want to play con or thm and have raises, cures, nukes, enfeebs, and -nas. you will stay in the class. But if you want to focus on healing, while completly ignoring nukes, you can turn on WHM job. This will let you heal and -na, and give you better abilities suited to healing, but will probably stop you from nuking. (vice versa with blm)

So if you are gla and you want to play the niche role of a pld instead of having the freedom of armory, you can turn on your pld job. This also means you're tied to you gla class level and weapon, while performing your role as main tank.