Yeah it's different than FFXI, not everthing is a job. Jobs are advanced specs but you can't cross class as much. For example, if you're a bard, which is a job that is an advanced Archer class, you can only use skills from Bard(duh), Archer and one more class, which in this case is Conjurer(heals and stuff, makes sense as a bard). You can't use other class skills. On the other hand if you stay an archer, you can use skills from any of the base classes, so you have more flexibility, to an extent. Also to start the quest for the job, you needed 30 in main class and 15 in secondary, but not sure if it's gonna be the same in the new FFXIV. So in this example to become a bard(which is done via doing a quest, similar to FFXI), you needed to be 30archer and 15 conjurer at least. They might change numbers or how it works though, not sure since I believe jobs aren't available until phase3. Finally, leveling as a job would also level the main class of that job(so your archer lvl would go up from lvling as a bard).