Typically an aftermarket cam will have more lift and duration... that is, it opens the valves more, and keeps them open longer.... allowing the motor to breathe better and thus perform better. That stated, depending on your riding you will want to get a cam that suits your needs. I run a Kitaco type 2 cam, its relatively mild (not tons more lift and duration than stock), I tried a TB cam (which is a copy of the DCR cam) which is pretty aggressive, and made the motor loose bottom end power and torque in favor for high rpm riding. For my needs i much preferred the Type 2 cam behavior.
There is much more to it but thats the general gist of it.