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.
This rogue anti-spyware application is often sold by means of misleading pop-up advertisements. One such case would be a pop-up ad that appears as a paid link when the search term "Microsoft antispyware" are entered into a search engine. Instead of taking you to Microsoft antispyware products list, it takes you to myspywarecleaner.com. This product is no longer in cirulation due to legal action from the Washington General Attorney.