Search engines help you find sites based on relevance to your search topic. For example, if you search on "Blogger Forum" (without the parentheses) you get a linear list of results based on the sites' importance.
Now, suppose that instead of a linear result you could get a 3-dimensional result. Suppose further that instead of a return based on a site's importance in relation to your search topic, you got a result based on the relationship of sites to not only your topic, but to each other. It is a lot easier to see what I mean than to try and explain it. So, here is the same "Blogger Forum" search, but done with Grokker: http://www.grokker.com/applet.html?query=blogger%20forum. Once you play with this for a bit, try entering your own search topics.
You will notice that you have relationships that are loosely connected and relationships that are within "neighborhoods" of related topics. You can see that Blogger Users is a neighborhood that includes Blogger Forum as a site and Blogger Family as a neighborhood that also includes Blogger Forum. However, the relationships within each neighborhood is a bit different. If you then click on one of the sites, the relationships change depending on the choices you make.
Warning: you may find you are spending hours investigating these relationships and discovering sites you never knew existed because a new relationship was suggested to you by Grokker.