A program which forwards HTTP, HTTPS and FTP requests, often but not necessarily caching them.

Squid is a widely used Free caching proxy server.

Some ISPs have "transparent proxies", where all web traffic gets redirected to a proxy without any knowledge (or choice) of the users. This is because in New Zealand, all international traffic incures data charges, so if the ISP can serve you a cached copy, they can save a couple of cents per megabyte.

Proxies are also frequently used in corporate networks, where FireWalls often block access to the outside world, and only proxies are allowed to communicate with it.

See also: ProxyServerNotes