Muggles' Guide to Harry Potter/Magic/Patronus

Overview
A Patronus is the visible result of the casting of the Expecto Patronum spell. By extension, it is also used to refer to the spell itself.

Extended Description
When the Expecto Patronum spell is cast, the result is an apparent object made of white vapour. This object, the Patronus, often takes the shape of an animal; Harry's, for instance, is a stag. The animal in question often has some relationship to the person casting it: for instance, Dumbledore's is a phoenix, which seems to fit him well given his choice of pet, and Harry's takes the shape that his father took as an Animagus. Dumbledore has also devised a way of allowing a patronus to carry messages. More information is available at Expecto Patronum.