I'm sure you can/will get much better answers than mine, but I'll give it a shot. There are many on the board here that can give you a very thorough answer.
As far as I know, the mob being killed is worth X amount of exp.
When mob dies, that exp is divided up amongst the group members, with higher level toons getting more of a share than lower level toons. A simple way of saying this is to add up the total levels in the group (say 3 level 10's, 2 level 8 and a level 7 for a total of 53). So the level 10's each get 10/53% (18.87%) of the exp, the 2 level 8's each get 8/53% (15.09%) of the exp and the lvl 7 gets 7/53% (13.21%) of the exp. Numbers are rounded, so this didn't account for .1% of the exp.
After that, each individual gets evaluated for different modifiers like full group bonus, zone exp modifiers, Veteran reward AA, exp potions, and race, which all get applied to your toon after the group split.
At this point I think you get the final exp you receive for that kill.