Butter is a dairy product, one of the most concentrated form of milk, made from churning milk or cream. During the process of churning, the liquid part (buttermilk) separates, leaving a solid part - mostly made fat and proteins. You can see this process with your own eyes by following the instructions in the How to make butter? section.
Butter is most commonly made from cow's milk, although you may also find butter from goat's, sheep's, buffalo's, or even yak's milk.
Surely you appreciate butter's phenomenal ability to add taste to everything it touches. You have almost 120 chemical compounds to thank for that! There are five crucial ones: fatty acids, lactones, methyl ketones, diacetyl, and dimethyl sulphide. Long live chemistry!