Caching Tutorial for Web Authors and Webmasters
www.mnot.net/cache_docs
What Jamie wrote
Thorough writeup on the various forms of caching to make websites optimal.
A Web cache sits between one or more Web servers (also known as origin servers) and a client or many clients, and watches requests come by, saving copies of the responses like HTML pages, images and files (collectively known as representations) for itself. Then, if there is another request for the same URL, it can use the response that it has, instead of asking the origin server for it again.