The Hilltop algorithm is an algorithm used to find documents relevant to a particular keyword topic. Created by Krishna Bharat while he was at Compaq Systems Research Center and George A. Mihăilă University of Toronto, it was acquired by Google in February 2003.
When you enter a query or keyword into the Google search engine, the Hilltop algorithm helps to find relevant keywords whose results are more informative about the query or keyword.
The algorithm operates on a special index of expert documents. These are pages that are about a specific topic and have links to many non-affiliated pages on that topic. Pages are defined as non-affiliated if they are authored by people from non-affiliated organizations. Results are ranked based on the match between the query and relevant descriptive text for hyperlinks on expert pages pointing to a given result page. Websites which have backlinks from many of the best expert pages are authorities and are ranked well.
Basically, it looks at the relationship between the "expert" and "authority" pages: an "expert" is a page that links to lots of other relevant documents; an "authority" is a page that has links pointing to it from the "expert" pages. Here they mean pages about a specific topic with links to many non-affiliated pages on that topic. If a website has backlinks from many of the best expert pages it will be an "authority".
In theory, Google finds "expert" pages and then the pages that they link to would rank well. Pages on sites like Yahoo!, DMOZ, college sites and library sites are usually considered experts.