Most people are familiar with the "six degrees of separation" theory which says that anyone on the planet is connected to anyone else through six people at most.
Some people may even be familiar with the "six degrees of Kevin Bacon" theory, which describes how close any Hollywood actor has been to starring in a movie with Bacon.
Now, it seems that the same thing is true about the web at large. Granted, it doesn't take just six links, but it seems that any two URLs on the web are at most 19 links apart.
This means you can start at any page on the web and end up at any other page with 19 clicks at the most.
One researcher, Albert-László Barabási, found that this is true because, even though most of the pages on the web are very weakly linked, i.e. only a few other pages link to them, there are large hubs, search engines in particular, that serve as a gateway between most documents or images on the web.