Most of the talk today is about having "authority" or "expert" pages link to your site in order to help your "themed" pagerank.
There is a lot of discussion about what is an "Authority" and I feel it is important to clarify the differences of authorities and hubs, plus show you how a search engine will use both concepts to rank a page.
Authority: What makes a page an authority? There is strong belief that an authority is determined by the value of their InDegree (an InDegree in our terms is the number of pages linking from page A to page B). The larger the number, the more authoritative that page is in terms of the types of themed pages that are linked to that page. A page within a theme is stronger based on the number of times a query term is found within the page.
This works just fine by itself for ranking pages if we did not have words with double or even triples meanings (windows, java, etc.). To make up for this factor we need something called "hubs" to help determine the best possible returned results for a keyword phrase.
Hubs: What makes up a hub? A hub can be defined as a site or page that has many links from pages that have the same links. So page A links to page B and page C, page B links to page A and page C, page C links to page A and page B and so on. The more similar and related links, the larger the hub is.
By looking at the authority of a page (InDegree) plus the hub or hubs the page is within (the similarity), the search engine can provide a better results page then just by looking at authority.
Most this information is from an article published by Jon Kleinberg while he as at Cornell. I tried to summarize to help us all understand the importance of what we are calling "authority sites".