Ransomware is a type of malware which restricts access to the computer system that it infects, and demands a ransom paid to the creator(s) of the malware in order for the restriction to be removed. Some forms of ransomware encrypt files on the system's hard drive, while some may simply lock the system and display messages intended to coax the user into paying.
Here are two window example:
Your PC is blocked due the violation of Copyright and Related Rights Law...
Cause of infections
These infections propagate through malicious advertisements (Malvertising) from streaming/downloads websites etc...
These ads lead to malicious exploits on websites to infect visitors.
Visitors running outdated software (Adobe Reader / Flash, Java) are prone to infection of these types.
Hence the importance of keeping up to date software.
A video of an exploit in action where the infection settles in simple visit to a Web site:
The malware deletes Safeboot keys of the computer.
Microsoft Standalone System Sweeper Tool is a tool provided by Microsoft that allows you to boot from a CD or USB stick and scan your computer with Windows Defender.This can be handy if Windows is locked by ransomwares/Trojan.Winlock.