One of the most important ingredients in all of the culinary arts, garlic nevertheless seems to defy, or transcend, attempts to define it in any but the most literal of terms. In that sense, then, garlic is a bulb, which like onions, shallots and chives, is a member of the lily family.
Intensely and uniquely flavorful and aromatic, garlic is used in virtually every cuisine in the world, including nearly every form of Asian, European, African, Latin American and North American cooking.
Garlic grows underground in the form of a bulb, from which long green shoots emerge. The garlic bulb is covered in a papery skin which is inedible. The bulb, or head, is in turn comprised of individual sections called cloves. These garlic cloves are themselves enclosed in the same paperlike skin, and the pale yellowish flesh within is the part of the garlic that is used in cooking.
Garlic has a powerful, pungent flavor when eaten raw. For that reason, it's customary to cook garlic in some way before eating it. Garlic is generally used as a flavoring ingredient in recipes rather than as the main ingredient itself, although roasted garlic can be eaten as a spread or condiment.
Garlic can be added to recipes that are sautéed, baked, roasted, braised; added to soups, sauces, marinades, spice rubs, stir-frys; minced and used in sausages, meatballs and other ground meat preparations.
There is probably no end to the uses and potential uses of garlic in the culinary arts. The green shoots of the garlic plant can also be eaten and have a garlicky flavor, though much less potent than in the cloves themselves.
So, what is garlic? Is it an herb? A spice? The truth is, it's neither. The word herb denotes something green, whether the leaves or stems of some sort of plant. The word spice indicates any other item, including roots, bark, seeds and so on, but specifically in the dried form. Garlic really doesn't fit either one of those categories.
So it's probably most accurate to call garlic a vegetable, even though it's hardly ever eaten on its own. In this sense garlic is most similar to onions and shallots, although ultimately garlic belongs in a category all its own.
Also see: Quiz: Is it an Herb or a Spice?