A padel racket unlike a tennis or squash racket, has no strings and is typically made of carbon fibre or fiberglass. Fiberglass is a lot cheaper to make, so you will find them in the lower end, beginner rackets. Carbon fibre is used in the more intermediate to advanced rackets.
One main differentiating characteristic of the padel racket is that it doesn't have strings. Premium padel rackets are made of high quality materials such as carbon, fiberglass, and Kevlar for the frame, and EVA or foam for the core.