A proxy or proxy server is a device that performs application layer caching, filtering, or processing in the middle of an end-to-end client-server connection. Proxies are often used for web or email content filtering. The origin of network traffic can also be disguised by routing an end-to-end connection through one or more proxy servers.
Proxies can be distinguished as either forward or reverse. For a forward proxy, the client is aware of the proxy and uses the proxy to obscure its address in making the request to the server. For a reverse proxy, the client is unaware of the proxy. The request is sent to what the client thinks is the true server. But instead, the reverse proxy acts in place of the true server and redirects the request to a server unknown to the client. Such reverse proxies can be used for load balancing and content distribution as well as for enhanced server security.
References:
http://en.kioskea.net/contents/lan/proxy.php3
http://searchsecurity.techtarget.com/definition/Web-filter