"Transfiguration is some of the most complex and dangerous
magic you will learn at
Hogwarts. Anyone messing around
in my class will leave and not come back. You have been warned."
-- Minerva McGonagall, first lecture to first-years (PS8)
Transfiguration is magic which changes one object into another. It is possible to change inanimate objects into animate ones and vice versa. Some Transfiguration spells alter a part of something, such as changing a person's ears from normal into rabbit ears. At Hogwarts, Transfiguration is taught by Professor McGonagall. Fifty years ago, Albus Dumbledore was the Transfiguration teacher at Hogwarts (CS13).
The opposite of Transfiguration is Untransfiguration, which would be returning something to its proper form (OP29).
Transfiguration is subject to rules, such as Gamp's Law of Elemental Transfiguration, which has five Principal Exceptions, of which food is one, which means that food cannot be produced out of thin air. Food can be moved from one place to another by magic, or changed, but not created out of nothing (DH15).
students try to change match into a needle (PS8)
students trying to turn a mouse into a snuffbox for their end of term Transfiguration practical - extra points for how pretty the snuffbox was, points off if it still has whiskers (PS16)
students turning a beetle into a button (CS6)
students turning pairs of white rabbits into slippers (maybe you have to be familiar with bunny slippers to think this is funny) (CS16)
student accidentally changes his friend into a badger in class (CS11)
fat white rabbit changing into a top silk hat and back in the Magical Menagerie (PA4)
Transfiguration practical: transforming a teapot into a tortoise, concerned over whether they had spouts for tails, breathed steam, or still had willow-patterned shells (PA16)
changing a hedgehog into a pincushion (GF15)
students changing guinea fowl into guinea pigs (GF22)
cross-species switches (GF22)