A clove of garlic is one segment of a head of garlic. A single head contains multiple cloves.
Garlic cloves range in size from very thin center pieces to fatter pieces on the outer edges of a head of garlic. The cloves are covered in multiple layers of thin, papery skin that must be removed before cooking the garlic, except in special circumstances noted in a recipe. Some cloves that are older have sprouts in the center that can be easily removed if you do not like the slightly bitter taste they add to the clove. Garlic presses are often used as a simple way of mincing or crushing individual cloves.