In fathom's example, three people started alink farm; each one added a new page to their sites that contains alink to all three sites. Then a fourth person joins the farm and adds an identical page of links to her site; all four web sites now contain pages with links to all four sites. This starts small and might even be undetectable by the search engines at first. But it can grow to thousands or even hundreds of thousands of people. By the time you geta link farm that big you have hundreds of pages in each web site that are all carbon copies of each other, and all pointing to all of those carbon copies. In short, it's a mess.