Most American butters (as well as most Canadian and British butters) are “sweet cream” butters, which means that they’re made from cream that hasn’t been allowed to sour at all. This was a big selling point in the days when dairy wagons frequently showed up late to collect cream from farms, by which point the product had fermented a bit. Butter made from soured cream was acidic and cheesy and mostly unpopular with American consumers. For that reason some dairies would “correct” the cream with an alkaline (like lye) which neutralized the acid but brought even more off flavors to the party. “Sweet cream” butter was bland, but far fresher tasting.
Sweet cream butters come in both salted and unsalted varieties. Since unsalted butter is so bland, even by American standards, natural cultured butter flavors are usually added to it. What are these “natural flavors”? Things like diacetyl, acetic acid, acetoin, ethyl formate, ethyl acetate, 2-butanone and other compounds that are created when natural lactic acid bacteria are allowed to go to work on milk or cream. Currently, unsalted butter accounts for only about 15% of the butter sold in America. 85% is salted.
In terms of its composition, American butters by law must be at least 80% butterfat and no more than 16% water. That last 4% is a mix of lactose, proteins and such. Salted butters contain 1-2% salt, which not only adds flavor, it acts as a preservative.
Feel like making a little of your own butter? Go here.