They live in households and help the people living there. They protect the family who lives on what the Brownie considers his property. They are fiercely loyal. They will except no gifts except for maybe scraps of food or a bit of milk, but anything more can anger a brownie. Shuffling sounds at night is a good indicator that a brownie is moving about. Brownies like to creatively organize things such as alphabetize a book shelf beginning with "Z". Brownies do not like laziness and if they are misused then they can turn into a boggart. Boggarts can turn back into brownies if you are nice to them.
* photos courtesy of Arthur Spiderwick's Field Guide