Flewwatt is a brand of heat tape as Jeff said. It is flat and therfore very usefull for reptile enclosures and the like. There are as many types of "heat tape" as one could imagine. For pipes it is usually called "heat tracing". In the process industry they use steam or glycol heat tracing as well as electric heat tracing to keep pipes from frezzing or to keep a certain temperature.
Flexwatt and other electrical resistance heaters just pass a current through an element that produces heat through electrical resistance.
UTH are easy because they come ready to go, out of the box. If you want to heat a large enclosure or many small ones (or if you just want to save some money) then you can make your own UTH out of Flexwatt heat tape. It will require you to wire it up your self.
If there is anything the nonconformist hates worse than a conformist, it's another nonconformist who doesn't conform to the prevailing standard of nonconformity.