Root beer is a North American beverage traditionally made using the root bark of the sassafras tree Sassafras albidum or the sarsaparilla vine Smilax ornata (also used to make a soft drink called sarsaparilla) as the primary flavor. It started out as a type of small beer that was brewed. Today, root beer is usually a sweet, carbonated soft drink that is generally non-alcoholic and caffeine-free. It usually has a thick and foamy head.

Source: Wikipedia