When you visit a webpage, the content (html files, pictures, flash, java scripts and other) is downloaded and saved to your computer. This is called the cache or Temporary Internet Files. Every time a cache entry is saved, it will also insert a pointer to the cache files in the index.dat file. Internet Explorer uses Index.dat files to keep track of offline data (the cache) like an index in the end of the book.

Internet Explorer uses the cache for two reasons. The primary reason is for fast access to web pages. If the web page you visit hasn't changed since the last time you visited, Internet Explorer will load the homepage from the hard drive (the cache) instead of the web. To find the cached homepage on the hard drive, it searches in the index.dat file and the browser shows the local page. The cache will also allow users to access homepages offline.