The gnome is a common garden pest resembling a potato with legs. Every so often, a garden must be “de-gnomed,” which involves grasping the gnomes by the ankles, swinging them around a few times to disorient them, then tossing them out of the garden.
Gnomes live in gnome-holes underground, where they dig up plant roots and generally cause a mess; their presence is also a dead give-away that a home belongs to a witch or wizard (Sch1). Gnomes are rather dim, so when they realize a de-gnoming is going on, they all come rushing up out of their holes to see what's going on, making them a lot easier to catch (CS3). Crookshanks loved chasing gnomes around the Weasleys' garden and the gnomes seemed just as much to love being chased (GF5).
One December when Fred Weasley was bitten by a gnome while picking carrots in the garden, he stupefied it and put it on the Weasley Christmas tree as "the ugliest angel Harry had ever seen, with a large bald head like a potato and rather hairy feet" (HBP16).
According to Xenophilius Lovegood, the scientific classification of the Gnome is "Gernumbli gardensi" (DH8). He believed the bite of a garden Gnome could bestow special gifts on a human, such as the ability to speak the language of Merpeople (Mermish) or suddenly sing opera. But to the rest of the Wizarding World they are merely a garden pest (FB).
The creature gnome has been popularized through modern culture as the garden gnome. A garden gnome is a is typically a small human-like creature with a long white beard and a pointy hat that people keep as decorations in their lawns. (Wikipedia)