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.

Form
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. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened.

The Patronus Charm, like Animagus forms, has been said to reflect the personality or feelings of the witch or wizard. They are, however, subject to change if the caster goes through an emotional change of some sort. For example, Nymphadora Tonks' Patronus changed from its previous form to that of a werewolf, reflecting her love of Remus Lupin, a werewolf. If a wizard is an Animagus and can summon a corporeal Patronus, the two may take the same form, as this was the case for Minerva McGonagall and James Potter I.

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 none other than Albus Dumbledore, who invented a way 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.

Etymology
Patronus means "protector" or "guardian" in Latin, reflecting the role the Patronus Charm plays. In archaic Latin, it meant "father", which is interesting, considering that Harry Potter's Patronus is the same as his father's Patronus and Animagus form. The Latin word exspecto or expecto means "I watch for" or "I await", thus the charm's incantation roughly translates into "I await a protector".

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, a stag, sent out a sort of omni-directional pulse, which drove the Dementors away, rather than running them down. In Harry Potter and the Order of the Phoenix (film), his Patronus physically attacks the Dementors as in the books.

Notes and references
Patronus Patronus