Server term can be used either for a computer or a software program that delivers data, information and software to other computers linked by a network. It handles requests for data, email, file transfers, and other network services from other computers (i.e., clients). Servers can be of various types Web, mail, and database servers are the most common servers accessed by the people around the world when using the Internet.
.A program or a type server that manages and maintains ad banners for a Web site or collection of Web sites. These programs are extremely sophisticated and are capable of keeping track and reporting Web site usage statistics. Ads can then be targeted towards certain types of individuals. It also provides the ability to rotate banners so a user won't see the same ad every time they come back to the same page.
Definition courtesy of www.netlingo.com