A proxy server is a server that acts as an intermediary between a workstation user and the Internet to ensure security, administrative control, and caching service.
A proxy server receives a request for an Internet service (such as a Web page request) from the user.
If it passes filtering requirements, the proxy server, assuming it is also a cache server, looks in its local cache of previously downloaded Web pages.
If it finds the page, it returns it to the user without needing to forward the request to the Internet.
If the page is not in the cache, the proxy server, acting as a client on behalf of the user, uses one of its own IP addresses to request the page from the server out on the Internet.
When the page is returned, the proxy server relates it to the original request and forwards it on to the user.