The Google Blog has a good explanation of the robots exclusion protocol.
- NOINDEX tag tells Google not to index a specific page
- NOFOLLOW tag tells Google not to follow the links on a specific page
- NOARCHIVE tag tells Google not to store a cached copy of your page
- NOSNIPPET tag tells Google not to show a snippet (description) under your Google listing, it will also not show a cached link in the search results
