Ransomware

Ransomware is a category of Malware seen when a Trojan Horse takes over a computer and encrypts the victim's drives, charging money to decrypt it, hence the "ransom".

First known case
In 1989 the AIDS Trojan was written, claiming to be an AIDS Info Diskette. When you ran the AIDSID, the trojan would encrypt your HDD and display a message demanding money in exchange for a password that decrypts the HDD and recovers your files, involving sending $189 to a Post Office box in Panama. If you did not pay, the drive would remain encrypted & you could not be able to access any files and/or programs.