Boggart

A boggart is an amortal shape-shifting non-being that takes on the form of the viewer's worst fear. When facing a boggart, it is best to have someone else along, to try to confuse it, since facing more than one person at once would make it indecisive as to what form it must take, usually a mixed-up amalgam of the victims' fears. Because of their shape-shifting ability, no one knows what a boggart looks like when it is alone, as it instantly changes into your worst fears when you first see it.

The charm that combats a boggart is Riddikulus. The charm requires a strong mind and good concentration. The incantation and wand movement alone will not affect a boggart. The correct way to perform the charm is to push past the fear, and concentrate on something that will make the boggart look amusing. The charm does not, in fact, repel a boggart; it just forces it to assume a shape that the caster will find comical, inspiring laughter, which will defeat a boggart.

Boggarts sometimes do not have the same strength or magic as what they imitate, for example, a Boggart transformed into a Dementor has weaker magic than a real Dementor or having a deadly scream as a true Banshee.

Muggles can sometimes see Boggarts, but rarely see them plainly and are often easily convinced that they are a figment of their imagination.

From Harry Potter Wiki: Boggarts

Hiding places

Boggarts are particularly fond of inhabiting certain places that are dark, such as in wardrobes, the gap beneath beds, and the cupboards under sinks and desks. On at least one occasion, a boggart was found hiding in a grandfather clock.

Famous boggarts

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 19th 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).

At Absit Omen

An infestation of boggarts in a seldom acessed area of the Hogwarts dungeons resulted in the brief and terrifying disaperence of some younger year students [1]