Magic is a natural force that can be used to override the usual "laws of nature", but is still considerably scientific in how it operates. It is what all witches and wizards in the Wizarding World use to do what they do. Must humans do not know that magic truly exists. There is a constant hiding from Muggles which must be done by witches and wizards.
Overview
In humans, the ability to perform magic or the lack such ability is an inborn attribute. It is the norm in the children of magical couples and rare in those of Muggles. Exceptions do exist: those unable to do magic who are born to magical parents are known as Squibs, whereas a witch or wizard born to Muggle parents are known as a Muggle-born, or the pejorative "Mudblood". The latter form are far more common than the rare Squibs, but this may be a feature of the disparate sizes of the Muggle and wizarding populations.
Other magical creatures in the wizarding world can also perform magic, such as house-elves and goblins. The House-Elf Kreacher demonstrated the ability to disapparate from the kitchens of Hogwarts where humans cannot. ("Elf magic isn't like wizard's magic, is it?" -- Ron Weasley.)
Using magic
For a person's ability to perform magic to be useful, a good deal of training is required to acquire the correct discipline. When 'wild,' typically with young and untrained children, it will still manifest itself subconsciously in moments of strong apprehension, fear or anger. A powerful or intelligent wizard can direct this force in less random ways, eg. Tom Riddle and possibly Ariana Dumbledore.
Almost all human magic is done with the use of a supporting tool or focus, typically a wand. One can do unfocused and uncontrolled magic without a wand. A few advanced wizards could perform directed magic without a wand.
A wizard or witch is only at their best when using their own wand. When using another's wand, one's spells are not as strong as they normally would be - due to the laws of Wandlore.
Limits
Regardless of how powerful a witch or wizard is, they are by no means without limits. For instance:
Rule of Conjuration: while it is possible to conjure things out of thin air, it is far more tricky to create something that fits an exact specification rather than a general one; moreover, any objects so conjured tend not to last.
Rule Against Resurrection: It is also impossible to resurrect the dead. Corpses can be transformed into obedient Inferi on a living wizard's command, though they are little more than zombies with no soul or will of their own. It is also possible via the rare Priori Incantatem effect to converse with ghost-like "shadows" of magically murdered people. The Resurrection Stone also allows one to talk to the dead, but those brought back by the Stone are not corporeal, nor do they wish to be disturbed from their peaceful rest.
Rule Against Immortality: Likewise, it is impossible to make oneself immortal unless one makes use of a mystical object of great power to sustain life (such as the Philosopher's Stone created by Nicolas Flamel or a Horcrux, the latter having been used by Lord Voldemort). If one were to possess the three Deathly Hallows, it is fabled that they would possess the tools to become the "master of death". However, being a true "master of death" is to be willing to accept that death is inevitable.
The Five Exceptions
There are five exceptions of the defined laws of what elements one cannot transfigure by use of magic. The five exceptions, are said to be: food, love, life, information and money.
Love Magic/Blood Magic
A loop-hole (or more) exists to prevent the immediate murder of a wizard. Blood seems to be one of them; Harry Potter's blood was used in Voldemort's resurrection; however, since Lily Potter's magical protection was in Harry's blood and his blood flowed through Voldemort's new body, Harry could not be killed while Voldemort was alive. He was, however, sent to limbo, where he was given the option of returning to life or moving on.
Magic and Emotions
A witch or wizard's emotional state can affect their inherent abilities.
- Nymphadora Tonks temporarily lost her power as a Metamorphmagus after suffering severe emotional turmoil when Remus Lupin would not return her affections. In effect, the form of her Patronus changed to reflect her love for him.
- As related to Harry by Dumbledore, Merope Gaunt only demonstrated any magical ability when removed from her father's oppression, but then seemed to lose it again when her husband abandoned her.
- Ariana Dumbledore (Dumbledore's sister), was emotionally scarred at a young age and thus her magic was volatile and uncontrolled.
- Harry Potter magically inflates his Aunt Marge out of sheer anger when she disrespects his parents.
Magic and Death
- Like love, death is studied in detail in a room (called the Death Chamber) of the Department of Mysteries containing an enigmatic veil (this suggests some sort of portal between the worlds of the dead and the living, but the exact significance of the veil is unclear). Sirius Black fell through this veil to his death.
- Magical techniques have been used to prolong life. The Philosopher's Stone can be used to prepare a potion that postpones death indefinitely.
- Lord Voldemort has availed himself of other methods, being one of the few wizards ever to use Horcruxes in his long sought attempt to "conquer death", and is believed to be the only one to use multiple Horcruxes.
- Also, the drinking of Unicorn blood will keep a person alive even if death is imminent, but at the terrible price of being cursed forever.
- Being magical can contribute to one's longevity, as there are several characters in the series who are quite long-lived - Albus Dumbledore, for example.
- It is revealed by Nearly Headless Nick that wizards have the option of becoming ghosts when they die. The alternative is "passing on". All Hogwarts headmasters appear in a portrait when they die, allowing consultation by future generations.
Magic and Love
Arguably the most powerful form of magic is also the most mysterious and elusive: love. Lord Voldemort, having never experienced love himself, underestimated its influence—to his detriment. It was through love that Lily Potter was able to save her son Harry from death by sacrificing her life so that he might live. Harry used very much the same mechanism to negate the power of Voldemort's spells against the students and teachers of Hogwarts. The exact nature of how "love-magic" works is unknown; it is studied in-depth at the Department of Mysteries where they have a giant cauldron of Amortentia.
The Wizarding World
The wizarding society exists as a shadow society to the Muggle world and works as hard as it can to keep its existence a secret, save for all but a few Muggles, such as those who are related to witches and wizards, or important Muggles such as the Prime Minister. To most magical people the Muggle world is unknown, and their attempts to disguise themselves as Muggles often have mostly humorous results. Muggle Studies at Hogwarts is considered a soft option. Most things of magical nature are hidden or otherwise obscured from Muggles; others (such as Dementors) simply cannot be seen by them, but Muggles do feel the effects of them. There is also an office in the Ministry of Magic, for the Misuse of Muggle Artifacts, that deals with people charming objects typically found in a Muggle society.