The term "sweet butter" most often means unsalted butter, while the term "sweet cream butter" can mean either salted or unsalted butter. The best way to ensure the purchase of unsalted butter is to look for "unsalted butter" on the label of the butter. Sweet butter is simply butter that is made using the sweet cream of milk, as opposed to butter made using cultured or soured cream.
Most butter processed in the United States is made from sweet cream. Unsalted butter is most commonly used in cooking, allowing better control of salt content. A stick of butter typically has about 80 milligrams of sodium per tablespoon, or about 1/4 teaspoon per stick of butter. While salted butter is also used for cooking, using unsalted butter allows for a more mellow butter sweetness in a finished dish compared to salted butter.
Salt is added to butter to lengthen its shelf life. Acting as a preservative, salt lengthens the shelf life of butter about one month over unsalted butter. Salted butter and unsalted butter can be used interchangeably in recipes. If sodium intake is a concern, the salt in a recipe can be cut back to account for the added salt in the butter.