Q: If you were an Animagus, what kind of animal would you be?
A: I'd like to be an otter - that's my favourite animal. It would be depressing if I turned out to be a slug or something.
- JKR on Animagi (Sch2)
Animagus (an-i-MAH-jus OR an-i-MAY-jus)
"animal" L. animal + "magus" Pers. magic user
Ordinary Transfiguration of a human into an animal's shape results in the human having an animal's brain, and thus being unable to remember what he or she was supposed to be doing (QA1, although Draco Malfoy's experience in GF13 is worth considering since he was apparently able to retain enough of his faculties to try to reach the dungeons while in ferret form). Retaining one's human mental capacities while still taking on an animal's form is a highly advanced form of Transfiguration, and can go horribly wrong (PA18). Consequently, those who can master this Transfiguration are quite rare in the wizarding world; they are referred to as Animagi. More precisely, an Animagus is a witch or wizard who has the ability to Transfigure herself/himself into an animal at will while retaining her/his mental powers, such as the ability to think as a human.
The Animagus Transfiguration can be performed wandlessly, since Sirius was able to accomplish it in Azkaban when his own wand was long gone (PA19). It is possible for another witch or wizard who is sufficiently knowledgeable to force an Animagus to Transfigure from animal shape back into human shape; the incantation (if any) for the spell is not currently known, but it is accompanied by a flash of light around the target Animagus. If used on a true animal, the "resume natural shape" spell would have no effect (PA19).
An Animagus' physical condition as an animal reflects his or her physical condition in human shape. For example, Peter Pettigrew's rat Animagus form lacked a toe on one of his front paws because he had lost a finger (PA19). (It is not known how his artificial hand later appeared in his Animagus form.)
An Animagus retains his or her clothing, spectacles, and so on throughout the transformation, disappearing when the Animagus is in animal shape but reappearing when human shape is resumed. Sirius, for example, when Transfiguring into human shape for a meeting with Harry, Ron, and Hermione outside Hogsmeade, retained his wizard's robes (GF27). Possibly the Transfiguration process moves the clothing into wizard space while the Animagus is in animal shape, but we simply don't know the details of how this works. In some cases items such as spectacles may affect the animal's appearance. For example, both Minerva McGonagall and Rita Skeeter wear spectacles in human shape, and these are not only transformed with the Animagus but in these two cases form part of the resulting animal's markings (PA6, GF37).
An Animagus appears to have some ability to communicate with true animals, although it is not known whether an Animagus must be in Animagus form to do so. For example, Sirius was able to understand fairly complex concepts from Crookshanks during their alliance in Harry's third year, such as the fact that Crookshanks had stolen a list of Gryffindor Tower's passwords from a boy's bedside table (PA19). Peter Pettigrew has an affinity with rats, and was able to learn Voldemort's hiding place in Albania from their knowledge of a place in the forest that was deadly to small animals such as themselves (GF33). Whatever this form of communication is, it evidently is not speech; Sirius did not know Crookshanks' name until Harry, Ron, and Hermione told him, although he had been working with Crookshanks for months at that point (PA19).
The Improper Use of Magic Office maintains a register of all known Animagi. In the books this has been referred to only as "the register"; its formal name is unknown. The register is a matter of public record; Hermione Granger at age thirteen could consult it for the information therein, including the names of registered persons (PA19).
The register keeps a record of, for each known Animagus (GF26):
the kind of animal that Animagus transforms into
There are penalties for being an Animagus and failing to register with the Improper Use of Magic Office. It is not known precisely what the legal consequences are for an unregistered Animagus, but they are apparently rather severe, as the threat of exposure as an unregistered Animagus was enough to persuade Rita Skeeter to take a year off from her job as a scandal-mongering reporter (GF37).
(names unknown) six other registered Animagi, twentieth century (PA19)