Google automatically takes a “snapshot” of each page it crawls and caches it. This enables us to show the search terms highlighted on text heavy pages so users can find relevant information quickly, and to retrieve pages for users if the site’s server temporarily fails. Users can access the cached version by choosing the “Cached” link on the search results page. If you do not want your content to be accessible through Google’s cache, you can use the NOARCHIVE meta-tag. Place this in the section of your documents:
This tag will tell robots not to archive the page. Google will continue to index and follow links from the page, but will not present cached material to users.
If you want to allow other robots to archive your content, but prevent Google’s robots from caching, you can use the following tag:
Note that the change will occur the next time Google crawls the page containing the NOARCHIVE tag (typically about once a month). If you want the change to take effect sooner than this, the site owner must contact us and request immediate removal of archived content. Also, the NOARCHIVE directive only controls whether the cached page is shown. To control whether the page is indexed, use the NOINDEX tag; to control whether links are followed, use the NOFOLLOW tag.
Excerpt taken from Google Webmaster Info