Aaron Wall from SEO Book has one of the easiest to understand explanations of TrustRank, I have found to date. He says:
Human editors help search engines combat search engine spam, but reviewing all content is impractical. TrustRank places a core vote of trust on a seed set of reviewed sites to help search engines identify pages that would be considered useful from pages that would be considered spam. This trust is attenuated to other sites through links from the seed sites.
Gary Price at SEW Blog in early March wrote an entry named Combating Web Spam with TrustRank and then followed up that entry in late April with Google Trademarks: TrustRank & The Neighborhood Wide Web.
The paper's abstract and a link to the full document can be found at http://dbpubs.stanford.edu:8090/pub/2004-17.