The shape of the cam is an indicator as to how "aggressive" the cam is. A round wheel is like a soft cam while a hard cam looks more like an oval shape. If you go to the Easton archery webpage and do the arrow selection guide, it will ask you if you have a hard, medium, or soft cam on your bow. Next to this prompt are some small pictures to show you which is which.

I dont' know if this is always true, but the hard cam designs tend to be faster than a soft cam, though they have (generally) a harsher draw cycle.