It's hard to imagine cooking without garlic - it's one of those ingredients I always have in my kitchen and it's used in almost every cuisine around the world. In the not too distant past, though, there was a snobbery about it within some cultures: garlic was considered to be an ingredient used only by the lower classes and immigrants.
The fat, moist cloves grow tightly packed in what is called a bulb or head, and are covered with a papery skin that's usually removed before the garlic is used. Garlic doesn't release its pungency and flavour until the flesh is pierced, sliced or crushed.
When raw, the flavour of garlic can be harsh and overwhelming, so it should be used with discretion, except in dishes where you want a strong, garlicky flavour; although even in something like an aioli, two cloves are usually enough to flavour the garlic mayonnaise. When garlic is cooked for a long time and slowly, the flavour becomes mellow and sweet - in the classic French dish of chicken with 40 cloves of garlic, the leisurely braising process makes the garlic taste soft and gentle.
There are many varieties of garlic and they range from strong to mild. The so-called elephant garlic has cloves that can be the size of whole heads of regular garlic. The flavour of elephant garlic is gentle and sweet, and it's one type that is eaten raw.
When buying garlic of any type, press the heads firmly to feel for any soft spots, which indicate spoiling. The skin surrounding each clove should fit tightly; if it's loose, it means the garlic is old and starting to shrivel. Sometimes the garlic will start to sprout. You can plant the clove (which will eventually grow more garlic) or you can slice it in half and remove the sprout. Garlic should be stored out of direct sunlight but not in the fridge.