Roller cams can be solid or hydraulic. But there is roller and flat tappet. Flat tappet style cams, common in the older vehicles from the mid 80's and back have a cylindrical lifter with a flat bottom(it actually has a slight convex curve to it). The cam lobe is tapered in such a way that the side of the lifter contacts the cam(actually it should be floating on oil between it and the cam) and it turns in the bore to prevent the cam/lifter from rubbing in 1 spot and eating each other up. The thing with flat tappet cams, is that the rate at which the valve is opened(often termed how agressive the cam is) has limits, since if the cam lobe becomes too steep(agressive), the corner on the bottom of a flat tappet lifter will dig into it and they'll destroy each other. With a roller cam, the bottom of the lifter actually has a steel roller on it, like a wheel, that rolls across the cam lobe. This has several advantages, it eliminates the friction and wear that is common in flat tappet cams, and since the bottom of the lifter now has a wheel instead of an edge, you can use a cam with a lobe that opens the valve and closes the valve much more quickly(a more agressive lobe), and the damage associated with the increased side loading of flat tappets and the gouging of the lifter's edge on an agressive lobe is gone. Roller cams let you run a cam that is comparitively more mild than a flat tappet cam(ie less lift/duration) but because the lobe profile is much more agressive it will actually move considerably more air. The benefit of a roller cam is more power with better drivability, better mileage, less wear, and reduced friction. The only disadvantages is the heavy weight of roller lifters make valve control more important, rpm is limited and proper spring selection is of vital importance since roller cams will float more easily than flat tappet cams, they also cost more. But the benefits far outweigh the downsides. Just use the right springs and you shouldn't ever have any problems with it

 

wow that helps a lot. great explanation. thanks!