Patronus Charm

A Patronus is an insubstantial animal form protector created by the advanced Patronus Charm spell, and one way to defend against Dementors and certain other dark creatures. The spell requires the use of a wand and the incantation "Expecto Patronum". Presumably, specific physical gestures with the wand are also required but they are not mentioned specifically. There are some whose Patronus is fully formed (corporeal) taking the shape of fairly solid-looking animals; the Patronus of some also appear only as a wisp of silvery mist. It is a very complex charm and many qualified wizards have trouble with it. Harry Potter is one of the youngest known wizards to cast a patronus; he was taught how do so in 1993 at the age of thirteen by Remus Lupin. In a Dumbledore's Army lesson, Harry taught the members to use the charm.

The Charm
In order to cast this charm one must think of an extremely happy memory or idea, and then say the incantation; Expecto Patronum.

Uses
There are two known uses for the Patronus charm. The first use of The Patronus is that it can be used to drive off certain dark creatures such as Dementors and Lethifolds, and is the only known charm that will work against them.

The other use was devised by Albus Dumbledore who devised the idea of using The Patronus to deliver messages. This source of communication is believed to be exclusive to the members of the Order of the Phoenix. Using the Patronus for communication offers great advantages in security, as Patronuses identify the caster and are Dark-proof.

The Patronus in the films
In the film adaptation of Harry Potter and the Prisoner of Azkaban, Harry's non-corporeal Patronus is more like a shield. In addition, his corporeal Patronus, rather than running down the nearby Dementors, sends out a sort of omni-directional pulse, which drives the Dementors away. In Harry Potter and the Order of the Phoenix his Patronus physically attacks the Dementors as in the books.