Heres a shot of the leveling system... Im guessing you might be even able to get a third class... don't know yet.
The Yellow bar on the side is the class level, you put points into that pool to gain stat points (str, dex, HP and others) aswell as its required if you wish to reach the higher level skills.
The skills are the boxes and circles, the circles are passive and the boxes are active skills. Each has a given amount of levels, You only require a single level in the skill to progress to the next enhancment of that skill (follow the line) but the main class level must be at the skills level to achieve it.
This all must be done each time your character levels, you only get 3 points to spread between everything... so you really got to think about what you want to do.
If you do make a mistake there is an NPC in certain towns that will remove points from a skill at a cost but you cannot remove class points (whatever you put in the main class is perm).
Pretty deep for a click crazy game like this.