Purpose: A Patronus Charm is an insubstantial animal protector. The first and main use of the Patronus is to drive off certain Dark creatures such as Dementors and Lethifolds. To cast the charm, one must concentrate on a happy memory - a memory that the Dementor feeds upon. A non-corporeal Patronus only impedes Dementors, while a full-fledged one can drive them away. The other use was devised by none other than Albus Dumbledore, who invented a way of using the Patronus to securely deliver messages.
Incantation: Expecto Patronum
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.”
Fact: A Patronus that is fully formed, or corporeal, takes the shape of fairly solid-looking animals; a non-corporeal Patronus appears only as wisps of silvery mist. Harry Potter is one of the youngest known wizards to cast a Patronus; he was taught how to do so in 1994 at the age of thirteen by Remus Lupin. The Patronus Charm, like Animagus forms, has been said to reflect the personality or feelings of the witch or wizard. They are subject to change/may be influenced by external factors.