When it comes to making us fall for their scams, cybercriminals have a number of tricks up their sleeves. One of these tricks is to scare us into thinking that our computers or smartphones have become infected with malware to get us to purchase a fake application. The programs and unethical advertising practices hackers use to frighten users into purchasing rogue applications are called scareware.
http://robertsiciliano.com/blog/2013/10/22/what-is-scareware/