Garlic is native to Central Asia and northeastern Iran, and has long been a common seasoning worldwide, with a history of several thousand years of human consumption and use. It was known to ancient Egyptians, and has been used both as a food flavoring and as a traditional medicine.
The entire "head" is called a garlic bulb, while each segment is called a clove. There are about 10-20 cloves in a single bulb, give or take.
We now know that most of the health effects are caused by one of the sulfur compounds formed when a garlic clove is chopped, crushed or chewed.
This compound is known as allicin, and is also responsible for the distinct garlic smell.
Allicin enters the body from the digestive tract and travels all over the body, where it exerts its potent biological effects (which we'll get to in a bit).