A related subject to theming is the Hilltop algorithm that Google is believed to be using. The Hilltop algorithm finds so-called expert documents on the Internet that are related to a particular keyword topic. These “authority sites” are well-established sites that contain a lot of information on a particular topic and have plenty of inbound links from related web sites. The Hilltop algorithm emphasizes the voting power of authority sites. This means that a link from an authority site is worth more than a link from other sites. Authority sites often appear at the top of the search results.
While authority websites are a primary source of content, there is another kind of a website, which is considered almost equally important as authorities by some search engine algorithms. These websites or website documents are known as hubs. Unlike authority websites, hub websites are collections that point to high-quality guides and resources that exist on other websites. Basically, hubs direct users to authority documents on a particular subject. Whereas authority websites often gain good rankings in the search results because of high link popularity, well-established hub websites can also be found at the top of the results in many occasions. Unlike with authority sites, this is not due to great link popularity. The hub websites rank so high in the search engines because search engines find hubs very useful for the users and as a result, hub websites are given a boost in the search rankings. Therefore, it is not only the incoming links that are valuable for a website, but also the outgoing links to good resources. (Kleinberg 1999.)