For those unfamiliar with the Harry Potter universe (or even hate it), an Animagus is a wizard who is able to assume the shape of an animal. Except for the shape and looks, the wizard has all human properties, like human intelligence, lifespan etc. (In the first chapter Uncle Vernon was surprised to see a cat read a map (which was in fact Minerva McGonagall who was in her Animagus form at the time), since cats can't read, but since this cat was actually a human in disguise, she could).