In the Harry Potter universe, an animagus is a wizard who can transform into an animal. The magic required is quite complex, and it can go badly if the wizard is not experienced enough to handle the transformation. As a result, the Ministry of Magic regulates animagi, requiring them to register themselves, although a number of unregistered animagi are clearly documented in the books. While in animal form, the animagus retains many human abilities, but also becomes more animal like.
The form which an animagus takes is not determined by the individual wizard. Rather, the animal form reflects part of the wizard's inner nature. Throughout his or her lifetime, an animagus can apparently only take one animal form, although it is possible that this may perhaps change under immense emotional stress, just as a Patronus charm does.
The extremely advanced magic does not require the use of a wand, as demonstrated by several transformations accomplished without one in the Harry Potter books. In addition, the wizard's clothing appears to be retained somewhere while he or she is an animal, because wizards are garbed again when they return to human shape. In the third book of the series, a charm which forces a wizard to assume human form is demonstrated. Should the charm be used on a real animal, the animal will not be affected.
It appears that while an animagus is in animal form, he or she may be able to communicate with animals. This is demonstrated in several of the books, when animagi gather or exchange information with other animals. The brain and cognition processes of the wizard also appear to be slightly slowed. This distinguishes an animagus from a wizard who has been transfigured into animal shape, as transfiguration appears to cause the wizard to have an animal brain.
When an animagus registers with the Ministry, he or she must provide information about the animal form taken and its markings. Presumably this helps the Ministry track animagi to ensure that they are not engaging in illegal activity. The only known registered animagus in the books is Minerva McGonagall, who turns into a tabby cat. Numerous unregistered animagi including Sirius Black, James Potter, and Rita Skeeter are also documented, and there may well be more. In all of these cases, the animagus has an advantage which legal registration would destroy; for example, Rita Skeeter transforms into an insect to spy on potential article subjects.