Proxy Servers are probably the most often used instances of a Proxy for most of our purposes.
Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST implement both the client and server requirements of RFC 2616.
A "Transparent Proxy" is a Proxy that does not modify the request or response beyond what is required for Proxy authentication and identification.
A "non-transparent proxy" is a Proxy that modifies the request or response in order to provide some added service to the user-agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP Proxy requirements apply to both types of proxies.