Ransomware is a type of malicious software that blocks access to the victim's data or threatens to publish or delete it until a ransom is paid. Any action is possible once a device or system is infected and there is no guarantee that paying the ransom will return access or not delete or publish the data. Simple ransomware may lock the system in a way which is not difficult for a knowledgeable person to reverse. More advanced malware uses a technique called cryptoviral extortion, in which it encrypts the victim's files, making them inaccessible, and demands a ransom payment to decrypt them. The ransomware may also encrypt the computer's Master File Table (MFT) or the entire hard drive. Thus, ransomware is a Denial-of-Service attack (DoS attack) that prevents computer users from accessing files since it is intractable to decrypt the files without the decryption key. Ransomware attacks are typically carried out using a Trojan that has a payload disguised as a legitimate file.
Source: Wikipedia, July 2017