One of most frustrating events you can experience as a webmaster or writer is finding your work has been copied without your permission. I'm sure that just about every writer and every webmaster has been horrified to find his own work somewhere else under a different person's name. The thankfully few times it has happened to me I felt a mixture of blind fury and complete hate.
Sometimes thieves don't stop there. They don't steal a web page or two and claim it as their own (this is merely a copyright violation and a completely unethical thing to do). No, what they do is steal a web page and claim it is YOURS, but with modifications. In other words, they create a web page which is exactly like yours, with some changes to do something undesirable.
Once they have added your page to a different site and made their changes, they submit it to search engines, advertise it in ezines and do all of other standard promotional techniques. They may also register similar domain names to try and fool people into going to their illegal site. Their purpose is to steal your traffic, directing it instead to their own web site (copies of your pages).
Why do they do this? Well, let's say you have a page which is attracting a heck of a lot of visitors. You are making quite a bit of money from affiliate links on that page. An unethical person might make a copy of that page on their own web site, and replace all of your affiliate links with his. Anyone clicking on those links would be generating money for pagejacker, not you.
Another common thing done by pagejackers is to add dozens or even hundreds of links to pornographic sites, many of which pop up automatically. Each time one of these links is displayed pagejacker gets paid a small amount, so more popups they display more money they make.