Authorities are those web pages, which contain relevant and exhaustive information on the topic and are considered to be an online 'authority' on the topic. Hubs are websites, which actually link to these authorities. The hub and authority model is a direct result of how the interlinking on the internet happens in a democratic way.

The following figure shows an example of how some (hypothetical) pages on the topic of SEO, organize themselves into hubs and authorities.

Figure 1: Hub-Authority Model of a set of sites on SEO. The arrows represent hyperlinks. The column on the extreme left has the hubs, the authorities are in the middle column and the extreme right contains a site mostly unrelated to our 'SEO' cluster.

As you can see in the above figure, most of the hubs have links to the authority sites. Moreover, almost all of them link to the same authority sites, and in fact this is how the authority sites are identified. The actual algorithm to find the hubs and authorities proceeds in an iterative fashion.

The process begins by first identifying sites related to the search term, SEO in this case. Related sites include sites which have content related to SEO and those which are linked by, and link to, such sites. Subsequently, the sites are ranked on their hub value and authority value in an iterative manner. The details of this iterative process is beyond the scope of this tutorial. Although, it should help to understand that a link from SEOForum.com to SEOTutorial.com will result in the hub value of SEOForum.com incremented by 1 and the authority value decremented by the same, and vice-versa for SEOTutorial.com. After the end of this process the hubs will be identified as sites with a relatively high hub value and authorities with a relatively high authority value. Note that sites such as STORA-ENSO-JOY.com shown above, which are unrelated to the hub-authority cluster, will neither have a high hub value or authority value.

The SEO lesson to be learned from this model is that web sites aspiring to have high search rankings in Google should try to link to almost all the authorities. Additionally, they should try to get as much inbound links as possible from sites with related topics.

There is a lot more to Google's ranking algorithm than the Hub-Authority model. Over the years, Google has evolved techniques to fight search spamming. Most of these techniques work by analyzing abnormalities in inbound site linking patterns. The full range of techniques employed by Google can be found in the recently released Google Patent Application. Read on for a detailed analysis of all the techniques contained in the Google Patent Application.