Short for Spam URI Real-time Block Lists, it is used to detect spam based on message body URIs (usually Web sites). SURBLs are not used to block spam senders. Instead they allow you to block messages that have spam hosts that are mentioned in message bodies. In order to use SURBL you need software that can parse URIs in message bodies, extract their hosts, and check those against a SURBL list.
See also RBL.