The Sun Java System Web Server uses a file cache to serve static information faster. In the previous version of the server, there was also an accelerator cache which routed requests to the file cache. The accelerator cache is no longer used. The file cache contains information about files, and static file content. The file cache also caches information that is used to speed up processing of server-parsed HTML.
For more information, see the online Performance Tuning and Sizing Guide on http://docs.sun.com.