Peeves is a poltergeist living in Hogwarts. He's a spirit rather than a physical being, but highly different from the ghosts he's occasionally mistaken for. Peeves is essentially an embodiment of disorder, and is constantly causing it, with varying amounts of mischievousness and malice. In appearance he's a small old man dressed in wildly coloured clothing. He's capable of flight and intangibility, like ghosts, but can also affect solid objects and make himself invisible.
He's usually far more annoying than legitimately dangerous, but acts in a wildly unpredictable manner and should not be given the chance to cause serious harm. It appears that his entire purpose is to cause disorder, break things, and be as annoying and disruptive as possible.
Peeves does not listen to Hogwarts prefects, teachers or anyone else, with the exception of Dumbledore and for some unknown reason the fearsome Bloody Baron. Argus Filch, who is usually left with cleaning up the mess and the damage Peeves causes, is his nemesis and works continuously to try to get Peeves thrown out.