I drew up a chart, wherein one starts classless (center of the chart), but in the way one levels up ones character it also calculates what "class" you are developing your character into. There are three main categories: WIZARD, WARRIOR, ASSASSIN. The chart is triangular (three corners, three main categories). There are many sub classes developing towards the corners and many between-classes if one is mixing it up (WIZARD with some ASSASSIN skill, etc.), and if one masters all three there are graduating Master classes.
I put the chart as my avatar, hopefully you can read it.
Edit: here's my attempt to paste the picture into this post: