The specialization system is generally covered as a different job - you just have to learn it from scratch (or level 40 or so depending). So when you want to DPS, instead of playing a Paladin, you play a Samurai, or maybe a Dragoon. it varies.

Personally, I wish there were a class that let you do DPS damage, and still have a shield - in WoW or FF. My Paladin is fun, but he's kinda slow at killing things compared to other classes. But that's a discussion for another time. :-)