Cultured butter is a European-style butter made with cream that has been cultured (fermented) with lactobacillus bacteria and then churned. It can be thought of as a butter made with a very creamy and rich yogurt.
Cultured butter has a tangy and rich flavor that is especially good when eaten directly on baked goods. It is not recommended for unmodified baking recipes because it will change the acidity of the batter.