From the Story
Boggarts are shape-shifters. They take the shape of whatever they think will frighten their audience most. They like dark, enclosed spaces such as wardrobes and cupboards. No one knows what Boggarts look like when they are alone, as they do not assume a form until they know what fear to emulate. It is always best to have company when facing a Boggart, as it will become confused deciding what shape to be. The charm used to repel a Boggart is 'Riddikulus', but the thing that really finishes a Boggart is laughter, so the spell should be used to force the Boggart to assume a shape that the audience finds amusing.
New from J.K. Rowling
A Boggart is a shape-shifting creature that will assume the form of whatever most frightens the person who encounters it. Nobody knows what a Boggart looks like if nobody is there to see it, although it continues to exist, usually giving evidence of its presence by rattling, shaking or scratching the object in which it is hiding. Boggarts particularly like confined spaces, but may also be found lurking in woods and around shadowy corners.
The more generally fearful a person is, the more susceptible they will be to Boggarts. Muggles, too, feel their presence and may even glimpse them, although they seem less capable of seeing them plainly and are usually easily convinced that the Boggart was a figment of their imagination.
Like a poltergeist, a Boggart is not and never has been truly alive. It is one of the strange non-beings that populate the magical world, for which there is no equivalent in the Muggle realm. Boggarts can be made to disappear, but more Boggarts will inevitably arise to take their place. Like poltergeists and the more sinister Dementors, they seem to be generated and sustained by human emotions.
The spell that defeats a Boggart can be tricky, because it involves making the creature into a figure of fun, so that fear can be dispelled in amusement. If the caster is able to laugh aloud at the Boggart, it will disappear at once. The incantation is ‘Riddikulus’, and the intention is to force the Boggart to assume a less-threatening and hopefully comical form.
Famous Boggarts include the Old Boggle of Canterbury (believed by local Muggles to be a mad, cannibalistic hermit that lived in a cave; in reality a particularly small Boggart that had learnt how to make the most of echos); the Bludgeoning Boggart of Old London Town (a Boggart that had taken on the form of a murderous thug that prowled the back streets of nineteenth-century London, but which could be reduced to a hamster with one simple incantation); and the Screaming Bogey of Strathtully (a Scottish Boggart that had fed on the fears of local Muggles to the point that it had become an elephantine black shadow with glowing white eyes, but which Lyall Lupin of the Ministry of Magic eventually trapped in a matchbox).