Cryptozoology is the study "hidden animals" (cryptids). These hidden animals are not accepted by science, but are substantiated by people's sightings, and their existence may already be accepted in the region where they are seen. These hidden animals are of three main groups: species that are new to science, surviving individuals of species thought to be extinct, and animals found in areas outside of their accepted range. Thus, a hidden animal could include something like a living dinosaur, or something as mundane as a clam unknown to science.
Though cryptozoology is often not accepted by most other scientists, and is a legitimite science. The creatures it deals with are not monsters, as many people refer to them, but animals that are unknown to science. Thus, cryptozoology does not deal with ghosts, demonic possession, paranormal and psychic phenomena, or supernatural phenomena (and to my knowledge it does not include UFOs or extraterrestrials).
The word cryptozoology was coined by Bernard Heuvelmans, "the father or cryptozoology".