If we are looking at WEB PAGE banner advertising.
A 'sender' is a program which targets specific IP address to send specific advertising to.
Messages that IP 'Senders' send are not considered spam. These messages do not use e-mail addresses or e-mail servers of any kind, and are broadcasted directly over the Internet but from a 'server' directly to a specific computer. 'Senders' are manufactured/sold by many software companies. (eg. XYZ Group)