cam diff.

ok now i have an engine with a roller cam in it, an i asked on a n other thread is i can use a none roller cam with the roller lifts and the answer was no, now i have not taken my engine completly apart so i have not seen a roller cam upclose, but what is the difference between a normal cam and a roller cam, or for that matter what is a hydraulic cam or a hydraulic roller can, from the not so close-up pics i have seen they all look the same, i just thought the lifters are what made the difference.

Corner of the lobes..