solid and hydraulic flat tappet cams look like have a flat bottom that rides on the cam lobe.
http://www.jegs.com/photos/817rhlifters.gif
Hydraulic lifters require no periodic adjustment to the valvetrain. They have an internal adjustment if you will, that compensates for valve lash. Set 'em once and you're done.
Solids have to have a clearance set between the rocker arms tip and the valve stem. This has to be done periodically. These rev higher than a hydraulic cam.
Rollers, as the name implies have a small wheel that rides on the cam surface.
http://www.jegs.com/images/products/...3016_small.gif
These also come in hydraulic and solid with the same advantage/disadvantage as a flat tapett cam.
With the roller tip you have less friction at the cam/lifter interface. The roller also allows for a more aggressive cam lobe profile. In other words the ramp that opens the valve can be steeper and get the valve open faster because of the roller.