Discovered in Book 3, Chapter 12, Expecto Patronum
"The Patronus Charm is a highly advanced piece of magic, well beyond Ordinary Wizarding Level standard. When performed correctly, the spell produces a Patronus, a kind of guardian which acts as a shield between a wizard and a Dementor. A Patronus is a positive force, a projection of the things that Dementors feed upon - hope, happiness and the desire to survive - but as it cannot feel despair, it cannot be harmed by a Dementor. Each Patronus is unique to the wizard who conjures it. It is conjured with an incantation, Expecto Patronum, but will only work if the wizard is concentrating properly on a single, happy memory. It is a hugely difficult to perform, and many qualified wizards have difficulty with it."
New from J.K. Rowling
"The Patronus is the most famous (and famously difficult) defensive charm. The aim is to produce a silvery-white guardian or protector, which takes the form of an animal. The exact form of the Patronus will not be apparent until the spell has been successfully cast. One of the most powerful defensive charms known to wizardkind, the Patronus can also be used as a messenger between wizards. As a pure, protective magical concentration of happiness and hope (the recollection of a single talisman memory is essential in its creation) it is the only spell effective against Dementors. The majority of witches and wizards are unable to produce Patronuses and to do so is generally considered a mark of superior magical ability.
Some witches and wizards may manage an incorporeal Patronus, which resembles a mass or wisp of silvery vapour or smoke. In some cases a witch or wizard may choose to produce an incorporeal Patronus deliberately, if he or she wishes to disguise the form it generally takes (Remus Lupin, for instance, is afraid that his corporeal Patronus gives too much away). The incorporeal Patronus is not a true Patronus and while it will give limited protection, it cannot provide the defensive power of the corporeal Patronus, which has the form and substance of an animal.
The Patronus Charm is one of the most ancient of charms and appears in many accounts of early magic. In spite of a long association with those fighting for lofty or noble causes (those able to produce corporeal Patronuses were often elected to high office within the Wizengamot and Ministry of Magic), the Patronus is not unknown among Dark wizards. While there is a widespread and justified belief that a wizard who is not pure of heart cannot produce a successful Patronus (the most famous example of the spell backfiring is that of the Dark wizard Raczidian, who was devoured by maggots), a rare few witches and wizards of questionable morals have succeeded in producing the Charm (Dolores Umbridge, for example, is able to conjure a cat Patronus to protect herself from Dementors). It may be that a true and confident belief in the rightness of one's actions can supply the necessary happiness. However, most such men and women, who become desensitised to the effects of the Dark creatures with whom they may ally themselves, regard the Patronus as an unnecessary spell to have in their arsenal.
No reliable system for predicting the form of an individual's Patronus has ever been found, although the great eighteenth-century researcher of Charms, Professor Catullus Spangle, set forth certain principles that are widely accepted as true.
The Patronus, asserted Spangle, represents that which is hidden, unknown but necessary within the personality. 'For it is evident,' he writes, in his masterwork 'Charms of Defence and Deterrence':
'... that a human confronted with inhuman evil, such as the Dementor, must draw upon resources he or she may never have needed, and the Patronus is the awakened secret self that lies dormant until needed, but which must now be brought to light...'
Here, says Spangle, is the explanation for the appearance of Patronuses in forms that their casters might not expect, for which they have never felt a particular affinity, or (in rare cases) even recognise.
Spangle is interesting on the subject of those unusual witches and wizards who produce a Patronus that takes the form of their favourite animal.
'It is my firm belief that such a Patronus is an indicator of obsession or eccentricity. Here is a wizard who may not be able to hide their essential self in common life, who may, indeed, parade tendencies that others might prefer to conceal. Whatever the form of their Patronus, you would be well-advised to show respect, and occasionally caution, towards a witch or wizard who produces the Patronus of their choice.'
The form of a Patronus may change during the course of a witch or wizard's life. Instances have been known of the form of the Patronus transforming due to bereavement, falling in love or profound shifts in a person's character. Thus Nymphadora Tonks's Patronus changes from a jack rabbit to a wolf (not a werewolf) when she falls in love with Remus Lupin. Some witches and wizards may be unable to produce a Patronus at all until they have undergone some kind of psychic shock.
It is usual, but not inevitable, for a Patronus to take the form of an animal commonly found in the caster's native country. Given their long affinity with humans it is perhaps unsurprising that among the most common Patronuses (although it must be remembered that any corporeal Patronus is highly unusual) are dogs, cats and horses. However, every Patronus is as unique as its creator and even identical twins have been known to produce very different Patronuses.
Extinct Patronuses are very rare but not unknown. Strangely, given their long connection with wizardkind, owl Patronuses are unusual. Most uncommon of all possibly Patronuses are magical creatures such as dragons, Thestrals and phoenixes. Never forget, though, that one of the most famous Patronuses of all time was a lowly mouse, which belonged to a legendary young wizard called Illyius, who used it to hold off an attack from an army of Dementors single-handedly. While a rare and magical Patronus undoubtedly reflects an unusual personality, it does not follow that it is more powerful, or will enjoy greater success at defending its caster."