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.
Privacy Defender is a Rogue anti-spyware application to remove spyware from users computers.
It always shows the same set of Registry keys and Cookies as Spyware detections, prompting user to buy license for Privacy Defender.
The deceiving Scan results always show the same set of Malware infections, even on a clean machine.