An Under Tank Heater is basically a heating element encased in laminated plastic. These usually work best with glass tanks and plastic tubs. These are good for providing belly heat for primarily ground dwelling snakes such as Ball Pythons, Dumerils Boas, Sand Boas, etc...

Radiant Heat Panels (RHP) are basically a heating element encased in a ceramic or fiberglass type core. These are mounted inside the enclosure to the top. They are normally used in PVC, Melamine, and custom built enclosures that have solid tops and front access. They are supposed to simulate heat from the sun and work best for arboreal and semi-arboreal reptiles that spend alot or most of their time basking on branches or perches. Since most PVC and Melamine enclosures are 12-14 inches tall, Ball Pythons can live in these with overhead heat. Even though they are mounted inside the enclosure, snakes can't lay on them for extended periods like they would with a UTH. I've never heard of a snake getting burned by an overhead RHP...