Google supplemental results

According to Google's FAQ page, supplemental results are part of Google's auxiliary index (main results are drawn from the main index) and pages, which appear on the supplemental listing, have "fewer restrictions" than those that appear on the main results page. They further say that the inclusion of sites on the main or supplemental index is purely automated and does not affect page rank at all.

In truth however, pages that appear on the main index will almost always show up first in a search. Supplemental search results will only show up if there are very few or no results at all in the main index. Plenty of older web sites also tend to populate the supplemental results page. Needless to say the supplemental results page is not where you want your site to end up. Ironically several people have emailed Google asking that their sites be included in the supplemental index!