Patronus Charm

"A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory… Allow it to fill you up…lose yourself in it…then speak the incantation ‘Expecto Patronum’."

—Remus Lupin teaching Harry Potter the 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, concentration on a powerfully happy memory, and the incantation "Expecto Patronum". Presumably, specific physical gestures with the wand are also required, but they are not mentioned specifically. The Patronus Charm is advanced magic and difficult to master, especially in emotionally difficult circumstances.

http://harrypotter.wikia.com/wiki/Patronus