A bogart or boggart is a creature of no known appearance that can take the form of anything those that look upon it are scared of. For example Ron Weasley who has a phobia of spiders would see it as a giant spider. Multiple attackers are not a problem for the bogart, it can become a chimera of multiple fears.
The way to defeat a bogart is with laughter, this can be done by using the reddikulus charm to turn it from something frightening into something funny.
Bogarts are used in third year Defense against the dark arts lessons.
Bogart a live in colonies along side bogeymen that they form a mutual relationship with. Both favour dark, damp places.
Another legend associated with Boggarts is that in some cultures they are less of a frightening shape shifter and more of annoying beast, hence they are sometimes called Bugbears because they like to bug people. This less ominous behaviour amounts to them essentially moving in with a family it finds amusing and annoying them. (Shape shifting into phobias is not known) according to some, to rid oneself of a resident boggart. You should spill flax seeds all over where the boggart is. Boggarts feel compelled to pick up all the flax seeds and eventually get bored and go to find fun elsewhere.