The boggart is a household spirit or hobgoblin (i.e., mischievous imp) of the English Folklore that can take numerous forms. The form it chooses depends of his mischievous intentions, however, whichever form they take, it is only rarely that they materialize, and therefore only specific boggarts will have a description.
Two rules of thumb –to remember if one suspects to be in the presence of a boggart— are never to up set a boggart, and to be specially weary of boggarts that can take human shape, for those are frequently more vicious than those materializing as animals
Boggarts are well known for playing tricks either in the household –where they delight in frightening people by pulling at bedclothes, rapping on and slamming doors, or snatching food from the children’s mouths— or by lurking on dark lonely roads or moorland, where they terrorize travelers by scaring their horses into hurtling, untying load from carts, or pretending to be an innocent bystander about to be run over. However, they can, if they do feel inclined, act much like a brownie helping with the heavy chores at home, washing, cleaning and doing heavy labor, but only if treated well, though once upset, this spirit will destroy or displace everything in the house and surroundings.
Once a boggart is upset it will attach itself to the household, making get rid of it practically impossible. Worst even, a sufficiently angered boggart will follow its chosen victims around during the day –which is perhaps why on certain days nothing seems to go right— and from house to house if the victims decide to move, make life as difficult as possible.
J. K. Rowling, in her Harry Potter series, reinterpreted the boggarts, turning them into shapeshifters that take the shape of that which is most feared by the person who encounters it, and that look like a dark blob, lurking in corners and empty spaces, when not in the sight of a person.
Briggs, K.M. 1957. The English Fairies Folklore, Vol. 68, No. 1 (Mar., 1957), Taylor & Francis, Ltd.