The proxy server checks to see if it has downloaded the document recently. If it has, you get a copy from the proxy's cache on its hard drive. At first it might seem that a caching proxy server would slow down surfing, but it doesn't. Often, there's only a small delay going through the server to get new material off the Net. And if the file's already in the cache, you save sending the data over the Internet entirely.
Web sites benefit from caching servers, too.
A large organization can set up a Web site and just download a single copy of a popular HTML page, rather than retrieving an identical copy for every person sitting behind the corporate firewall. That means a site can have a million readers a day but only have a thousand hits, which in turn means you don't need to have a whole farm of Web servers in order to serve up popular content. Most advertiser- supported Web sites are terrified of caching proxy servers. But most advertiser-supported Web sites are terrified of caching proxy servers. That's because there's no way to differentiate between a single reader and a hundred thousand when you're sitting at the other end of the cache without looking at its log files.