If you're really having trouble remembering them, just open up your own character's profile under 'Character' in the menu bar and click on 'Classes' - it lists every class and job with their icon and name.

Most of the class icons are pretty obvious if you remember what weapons they use, as that's all the icons (mostly) are - the mage ones are a little more obtuse however (like the celestial globe for ACN and a flaming urn for THM).