Animagus
Ordinary Transfiguration of a human into an animals shape results in the human having an animals brain and thus being unable to remember what he or she was supposed to be doing. 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. 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 her/ himself into an animal at will while retaining his/ her mental powers, such as the ability to think as a human.
The Animagus Transfiguration can be performed wandlessly. 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 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.
An Animagus' physical condition as an animal reflects his or her ohysical condition in human shape. For example, Peter Pettigrew's rat form lacked a toe on one of his front paws because he had lost a finger. An Animagus also 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. 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.
The "Improper Use of Magic" office maintains a register of all known Animagui. It is referred to as the register. The register is a matter of public record. The register keeps a record of, for each known Animagus, the kind of animal that Animagus transforms into and distinguishing markings. There are penalties for being an Animagus and failing to register with the "Improper Use of Magic" office.
Metamorphmagus
Metamorphmagi are born with the ability to change their appearance at will. The word comes from the Greek prefix "meta", meaning change, "morph", meaning form and "magus", meaning wizard. These are witches or wizards that are able to change his or her physical appearance at will, rather than requiring a potion or spell like the rest of the wizarding population. They are extremely rare.
Unlike Animagi, Metamorphmagi can take a wide variety of forms, changing gender, race and age or even just a part of teir appearances such as the shape of their noses. A Metamorphmagus is born, not made; it is not a skill that can be learned. There also seems to be a degree of heritability. The ability presents very early in life. It is unknown whether Metamorphmagi receive any training in developing their innate skill or if they simply learn through trial and error, but given their rarity, it is somewhat unlikely that much training is available.
It is likely that Metamorphmagi must register with the Ministry of Magic. Like many other types of magic, a Metamorphmagus' abilities are affected by his or her emotional state. The person's appearance at any moment will often reflect the emotions the person is feeling at the time. Strong emotions such as shock or depression, can affect their abilities to the point of having trouble using them at all.