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.
SpyHeal is a rogue antispyware which display false detections on user's machine and force them to buy the product.
Showing False Detections
From the SpyHeal website
Poor Uninstaller which does not remove Spyheal.exe from user's machine