A long-tongued custard-coloured little furball that makes a calming purring noise. These pleasant creatures are often kept as pets, particularly by wizarding children; Ron Weasley used to own one until Fred used it for Bludger practice. As scavengers that will eat anything from leftovers to spiders, they are very easy to care for.
A group of puffskeins is called a poffle (JKR).
If you immediately thought of "tribbles" from STAR TREK when you read about the "custard-colored furballs" in the Magical Menagerie, you're not alone. See the original series episode "The Trouble with Tribbles" written by David Gerrold for more information on that subject. As a matter of fact, David Gerrold got the idea for Tribbles from "flat-cats" in a short story by Robert Heinlein.