Ok Zendo Im guessing your a PS n00b then?
You want to check out the front page for this site.
http://www.planetside-universe.com
There are many links on the left hand side with certification info, empire info. The stuff you are looking for, plus the FAQ.
Also the official site has lots of info, but ours is better
PS doesn't have "classes" as such, but specialisations depending on what you decide to "certify" in. During your career as you level up you can get cert points, which you can buy certifications in such as armour, weaponry, skills. The only limit is the amount of cert points you have, but you can also uncertify (that takes 24hours real time).
So your not fixed like you are in EQ etc, but you are limited by the amount of certs you have to spend.