Google, Yahoo, Bing and other search engines store a cached copy of all webpages that are crawled and stored in their web index. The cached copy of the website is served to the visitor when the webpage is down, unavailable or cannot be reached, which is useful because users can still access the information in case the website is facing technical problems or is deleted forever. If you click on the “Cached” link, you will see the web page as it looked when Google or other search engines indexed the page in question. The cached content is the content Google uses to judge whether this page is a relevant match for your query.
For example: Let’s say you want to preview the cached version of this blog post. To do that, you have to use cache: URL of the webpage in Google search box and hit the “Return” key. Google will then show the latest cached version of the webpage in a new brower tab, an example can be found here
Quickly Open Cached Version of A Webpage From Firefox
Firefox users who often have to deal with cached copies of webpages can try the CacheMachine Add-on for Firefox to quickly open cached copies of any webpage in Firefox. The extension adds 3 handy links to Firefox’s right click context menu e.g Open cached version of a website with Google, Yahoo or Bing.
Clicking a particular cached link option opens the cached copy of the link in a new browser tab. This is handy on two occasions. First, when the webpage is down or not available and you want to open the latest cached copy of the site, without having to remember any operator. Second, when you want to read a site’s content from the cached copy, which is free from advertisements (see example)
CacheMyMachine is a free download, works with Firefox 3 and 4. [ via Ghacks ]