It's called an Oxford comma, you nonce. It separates the items on the list. For instance: 'Hall and Oates, and Madonna played tonight's concert.' That states that Hall and Oates are one item and Madonna is another. If the sentence were: 'Hall, Oates and Madonna' it would imply either two things; firstly that there are three individual acts, or that Hall performed and then Oates and Madonna performed.