Muggles' Guide to Harry Potter/Magic/Expecto Patronum

Overview
"This ancient and mysterious charm conjures a magical guardian, a projection of all your most positive feelings. The Patronus Charm is difficult, and many witches and wizards are unable to produce a full, corporeal Patronus, a guardian which generally takes the shape of the animal with whom they share the deepest affinity. You may suspect, but you will never truly know what form your Patronus will take until you succeed in conjuring it."— Miranda Goshawk (author of the Standard Book of Spells)

Expecto Patronum, or the Patronus Charm, will cast a Patronus, which can appear as simply white vapour, or in more advanced casters, as a silvery-white animal shape. If it takes the shape of an animal, it is called a corporeal Patronus. This spell is used to ward off Dementors, which are the guardians of Azkaban. In Fantastic Beasts and Where to Find Them we are told that this spell is also useful against Lethifolds, but we do not see them used this way in the main series.

Extended Description
"That's very, very advanced magic ..." — Hermione Granger on the difficulty and complexity of the charm

Harry learns this spell from Remus Lupin in Harry Potter and the Prisoner of Azkaban. Harry's ability to cast a corporeal Patronus in his third year of magical studies shows his ability as a wizard, as it is a very advanced spell which is thought to be too difficult for underage wizards, in fact many witches and wizards are unable to produce any form of Patronus. In his fifth year, when Harry takes his OWLs, his Charms examiner offers him an extra point if he can produce a corporeal Patronus. A point, or a letter grade, would be worth about 15% of his total marks, so achieving this one charm is seen as quite valuable.

The primary purpose of the Patronus, as mentioned, is as defence against Dementors. Professor Lupin describes it thus: "... a kind of anti-Dementor – a guardian which acts as a shield between you and the Dementor." And: "The Patronus is a kind of positive force, a projection of the very things that the Dementor feeds upon – hope, happiness, the desire to survive – but it cannot feel despair, as real humans can, so Dementors can't hurt it." Presumably, the Dementor, finding that the being confronting it does not suffer as a human would, retreats in confusion. In Harry Potter and the Order of the Phoenix we see Harry's Patronus charge at the Dementors; though the Patronus may have no physical body, the Dementor, being sightless, cannot see that, all it can sense is this emotional blast furnace, which is charging at it, and it will retreat to save itself.

We also see the Patronus used as a signaling spell when Professor Dumbledore sends for Hagrid in Harry Potter and the Goblet of Fire, and also when Tonks sends up a Patronus to summon someone from within the school to open the gates in Harry Potter and the Half-Blood Prince. According to the author, use of the Patronus as a communications device was pioneered by Dumbledore and is used exclusively by members of the Order of the Phoenix.

The Patronus is used extensively in Harry Potter and the Deathly Hallows, both as a communications device, as the Order of the Phoenix is now an outlaw organization, and also as defence against the Dementors who are wandering the English countryside. In particular, we see Arthur Weasley using it to warn Harry, on his birthday, of his imminent arrival with the Minister for Magic. The following day, Kingsley Shacklebolt sends his Patronus to warn the members of Bill and Fleur's wedding party that the Ministry had fallen; Arthur sends a Patronus later that night to tell Ron, Hermione, and Harry that the Weasley family is all right. We also see Patronuses used as defence against Dementors by Harry, Hermione, Luna Lovegood, Ernie Macmillan, Seamus Finnigan, and Dolores Umbridge. We see Aberforth Dumbledore's Patronus, a goat, when the Trio return to Hogsmeade in Harry Potter and the Deathly Hallows. Finally, we see Severus Snape's Patronus on two occasions.

The shape of a corporeal Patronus is significantly influenced by the personality of the caster. Harry's is a stag; Dumbledore's is a phoenix; Tonks' Patronus, as seen in Harry Potter and the Half-Blood Prince, is "something large and hairy", and has apparently changed recently. Members of Dumbledore's Army are also, some of them, able to conjure Patronuses by their last lesson: Cho Chang's is a swan, for instance, while Hermione's is an otter. Life events can change the shape of a Patronus as well; as mentioned, Professor Snape mentions that Tonks' Patronus had changed. This change apparently happened at the death of Sirius Black, and Harry initially thinks that Tonks' new Patronus has taken on Sirius' dog shape.

It is because of this connection between the caster's personality and the shape of the Patronus that it can also be used, to a certain extent, as identification of the wizard. Harry is identified, for instance, by his Patronus in Hogsmeade; the Death Eaters guarding against his return to Hogsmeade decide that Harry has arrived by the shape of the Patronus he uses to defend himself from the Dementors they send after him. This is only a weak identification, as it is possible for multiple wizards to have the same Patronus animal, and because not all wizards can successfully cast this spell; but it is unlikely that a wizard in disguise would have the same Patronus as the wizard he is trying to impersonate.

Literal Latin translation: I desire my patron.

Analysis
It should be noted that a sufficiently advanced wizard or witch can apparently cast multiple simultaneous Patronuses. We see that Professor McGonagall casts three Patronuses at once, in Harry Potter and the Deathly Hallows, when she needs to contact the three other Heads of House.

Questions

 * 1) Harry's Patronus is a Stag. His father's Animagus form is also a stag. We know from the description of the Animagus that the form the wizard takes when transformed is related to his personality. Are Patronuses and Animagus Forms related? Does this pass from father to son, and if someone's Patronus' form changed, would their Animagus form also change?
 * 2) Clearly, the form a wizard's Patronus takes is related to his personality or events in his life. If you had a Patronus, what shape would it take?
 * 3) If your Patronus was nothing like your Animagus, why would that be?

Greater Picture
There are two spells that relate a wizard or witch to an animal shape: the Patronus charm, and the Animagus transformation. In the course of the books, we see only one witch who is both Animagus and casts a visible Patronus, that being Professor McGonagall. The fact that her Animagus form is the same as her Patronus, a tabby cat, would indicate that whatever influences the shape of the one would also influence the shape of the other. It is perhaps a reasonable assumption that the shape resulting from either of these spells is related to the current mental state of the caster; but the Animagus transform, being a one-time enabling of the ability to transform, likely does not allow for change in the animal one changes to, rather enabling the transform according to the state of the caster at the time the transform was enabled, while the Patronus charm, being invoked afresh each time, reflects the caster's current state. There is more discussion of this at the entry for the Animagus transform.

While it is noted in the Extended Description that we see Professor Snape's Patronus on two occasions, we deliberately elide the shape of that Patronus as it constitutes a relatively large spoiler. Snape's Patronus takes the shape of a doe, and is seen first in the Forest of Dean, and later in Snape's memories of a conversation with Professor Dumbledore. Dumbledore recognizes it as being related to Lily Evans, as she then was, and Snape's unrequited love for her. We note that in three instances, Harry, Snape, and Tonks, the Patronus has taken a shape related to a major object of affection: Harry's is related to his father, Tonks' is related to Lupin, and Snape's to Lily. Clearly there is some indication that the Patronus form is influenced by love, requited, unrequited, or lost. Related to this we note that Hermione's Patronus is an otter, and if its form was related to love objects, we would expect it to be a weasel; Hermione, however, likely is aware that weasels and otters belong to the same family, Mustelidae.