In IIS 7, you can configure output caching to improve performance on your Web server, site, or application. When a user requests a Web page, IIS processes the request and returns a page to the client browser. If you enable output caching, a copy of that processed Web page is stored in memory on the Web server and returned to client browsers in subsequent requests for that same resource. This eliminates the requirement to reprocess the page every time that it is requested. This is helpful when your content relies on an external program for processing, such as with a Common Gateway Interface (CGI) program, or includes data from an external source, such as from a remote share or a database.

Note
Because cached items are retained in memory, they are dumped if resources run low on the server. The page will then be re-cached the next time that a user requests that resource, if the server determines that the page is sufficiently popular to be cached.