Server: A process that runs on a host that relays information to a client upon the client sending it a request. Servers come in many forms: application servers, web servers, database servers, and so forth.
From: http://www.loadbalancing.net/glossary.html

A powerful computer that holds data to be shared over a network. Can be used to store critical data for retrieval. A server also acts the communications gateway between many computers connected to it, responding to requests for information from client computers. On the Internet, all Web pages are held on servers. This includes search engine and directory data accessible from the Internet. Typically, the computers running the server software are dedicated to that purpose.
From : www.botics.com/marketing/seo_glossary.htm