Links from pages that have PageRank 0 still count as backlinks, but google doesn't think they as important as links from a page with a high PageRank. Nobody knows exactly what all the components to the google algorithm are because google doesn't tell anyone. They give general advice about the main factors, e.g http://www.google.com/support/webmasters/bin/topic.py?hl=en&topic=8522 but it's not in their interest to be too specific, as those nasty seo people would use that information to try to manipulate the results
For matching terms, Google uses LSI algorithms,this algorithm makes Google smart enough that he understand the semantics of terms used..