Taken directly from Google Help Center.
What's a "supplemental result?"
A supplemental result is just like a regular web result, except that it's pulled from our supplemental index. We're able to place fewer restraints on sites that we crawl for this supplemental index than we do on sites that are crawled for our main index. For example, the number of parameters in a URL might exclude a site from being crawled for inclusion in our main index; however, it could still be crawled and added to our supplemental index.
If you're a webmaster, please note that the index in which a site is included is completely automated; there's no way to select or change the index in which a site appears. Please also be assured that the index in which a site is included doesn't affect its PageRank. For tips on maintaining a crawler-friendly site, please visit http://www.google.com/support/webmas...y?answer=35769