Proxy servers provide a cache of items available on other servers which are presumably slower, more expensive to access or unavailable from the local network.
The process of proxying a network through a single host on another network is called network masquerading or IP-masquerading if the source and target networks use the Internet Protocol.
This term is used particularly for a World Wide Web server which accepts URLs with a special prefix. When it receives a request for such a URL, it strips off the prefix and looks for the resulting URL in its local cache. If found, it returns the document immediately, otherwise it fetches it from the remote server, saves a copy in the cache and returns it to the requester. The cache will usually have an expiry algorithm which flushes documents according to their age, size, and access history.
The Squid cache is the popular http proxy server in UNIX/Linux world. However, Apache's mod_proxy module also provides proxying and caching capabilities, and has the advantage of already being installed on nearly all systems.