These are usually anti-spyware or security software applications that use various forms of deception and/or unethical means or show a history of negligent false positives to goad the end user to make a purchase.
In some cases these applications maybe downloaded with some form of unwanted software at which point the rogue application is offered to the customer as a way to remove the unwanted software.
XPAntivirus main scan window showing that it has detected malware on a computer that is not infected.
Warning window asking to remove all threats found.
Window requiring user to enter in license key to remove the fake threats. If user does not have key, user has option to purchase a key to remove the fake threats.