Transfiguration

Transfiguration is the art of changing of the form and appearance of an object and the vanishing of objects. This magical art is governed by Gamp's Law of Elemental Transfiguration which acknowledges the limits to the power of this branch of magic.

Hogwarts School of Witchcraft and Wizardry devotes an entire section of their curriculum to Transfiguration.

Transfiguring can be done to most, if not all objects. Sometimes if it is done improperly the transfigured object can become half-transfigured or permanently stuck. Victor Krum half-transfigured himself into a shark in the Second Task of The Triwizard Tournament in 1994. However, he was not stuck in this form.

Animagi
Turning yourself into the animal that most represents you makes you an Animagus. This is considered a form of transfiguration, and a hard one at that. It takes years for one to master this skill but proves very useful. Minerva McGonagall was a registered animagus, but stated herself that it was only for educational purposes. However Animagi can transform at will, while you need a spell to transfigure oneself. Animagi can only take the form of one animal. This animal form is not chosen by the wizard, but determined by their inner traits and personality.