Patronus Charm

A very powerful charm that is taught at the NEWTs level is called the Patronus Charm. This charm is very advanced and at a difficult level. The theory of this charm is to wave your wand and say the incantation “Expecto Patronum!”. Although it may sound simple enough, actually casting the spell correctly is not. The caster must concentrate on the spell and a powerful happy thought. This makes the spell work. Since this spell is most known to drive away dementors, Azkaban guards, this is the reason why it is not so easy. Dementors suck away your happiness and hope, leaving you drowning in your sorrow and misery. It is very hard to conjure up a corporeal Patronus when you feel like life cannot go on. Although this charm also works on other dark creatures such as lenthifolds, which are dark creatures that tend to secretly slip into houses to suffocate victims to disgest as their next meal.
The word patronus in Latin means “guardian” or “protector” while the word expecto means “I watch for” or “I wait”. A Patronus comes whenever it is commanded to when the spell is cast and it starts to fade away after it’s use is not needed. When it is conjured correctly, a person’s Patronus becomes a silvery, glossy version of their inner animal that their personalities show. Usually a person’s Patronus stays for life and is not their choice. They do not change unless that person has faced a tramatic event or their personal feelings change, such as falling in love. A Patronus can be used to repel dementors but also as a signal or warning to carry messages. The Order of Phoenix uses Patronus to convey messages because a Patronus are always Dark wizard proof.
Some known examples of Patronus form deciding for themselves are Harry as a stag for his father’s Animagus form, Dumbledore as a phoenix (possibly for Fawks who he seems very close to), and Nymphadora Tonks as a werewolf because she fell in love with Remus Lupin.

Source: http://en.wikipedia.org/wiki/Patronus_Charm#The_Patronus_Charm