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.
A Windows Error fixing tool that asks the users to register it to fix the detected errors. May give overstated reports of defects found in user computer. Installs through improper bundling.