"So the boggart sitting in the darkness within has not yet assumed a form. He does not yet know what will frighten the person on the other side of the door. Nobody knows what a boggart looks like when he is alone, but when I let him out, he will immediately become whatever each of us most fears."
—Professor Lupin to his Third year class in 1993[src]

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.
A boggart is a shape-shifting creature 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. Because of their shape-shifting ability, no one knows what a boggart looks like when it is alone, as it instantly changes 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, according to Lupin.

He took the class to the staffroom, where a boggart had moved into a wardrobe the previous afternoon. After explaining what they would need to do, Professor Lupin let the class, starting with Neville Longbottom, perform the Riddikulus charm on the boggart. When the boggart moved towards Harry Potter, Lupin drew the boggart's attention, causing it to turn into a full moon, then had Neville finish it off.

Later, Professor Lupin explained that the reason he didn't let Harry face the boggart was that he thought Harry's boggart would turn into Lord Voldemort, and he didn't want to panic the students. Harry's boggart actually took on the form of a Dementor, as he had an encounter with them on the way to Hogwarts that year and heard his mother's dying moments whenever they came near.
Remus Lupin's full moon boggart.

To help Harry, Professor Lupin taught him how to perform the Patronus, using a boggart found in Argus Filch's filing cabinet as a substitute for a real Dementor. In between the private lessons, Professor Lupin kept the boggart in a cabinet under the desk, in his office.

On 24 June, 1994, Harry Potter came across a boggart in the Third Task of the Triwizard Tournament. When he first encountered it, Harry thought the boggart was actually a Dementor. He cast his Patronus, and realised that the creature was in fact a boggart when it fell back and tripped over the hem of its robes instead of being driven back, as a Dementor would have been.

"P-P-Professor McGonagall! Sh-she said I'd failed everything!"
—Hermione Granger faces her Boggart