Ransomware is a type of malware used by hackers to disable a computer or service with the goal of extorting money from the victim.
There are two types of ransomware, lockscreen ransomware and encryption ransomware, with the latter also being called crypto-ransomware.
Lockscreen ransomware disables a computer by displaying a full-screen message which prevents the user from using and accessing files. The message will normally employ social engineering techniques in order to threaten and scare the victim into paying the ransom.