What is the beast actually? How is it described?

THis all depends on the interpretation. In chapter 6, Sam and Eric see the dead parachute man from afar and fill in the details themselves. They interpret the man to be the beast with "claws" and "sharp teeth". So the physical manifestation of the beast is a dead pilot hanging somewhere on top of the mountain. Psychologically the beast simply exists within each boy. Simon catches on to this early on but this truth remains lost on most of the boys.