Ransomware is a type of malware that restricts access to a computer system that it infects in some way, and demands that the user pay a ransom to the operators of the malware to remove the restriction.
Locker ransomware: typically spread through social engineering, phishing campaigns, and watering-hole sites.
Crypto ransomware: Instead of restricting user action by denying access to the user interface, Crypto ransomware targets the data and filesystems on the device. Crypto examples include: Locky, TeslaCrypt/EccKrypt, Cryptolocker, Cryptowall, CTB-Locker...
Ransomware is honed in on small-to-midsize businesses (SMBs) and distributed enterprises. In fact, 42% of SMBs consider crypto-ransomware to be the most significant threat they face today, in large part because they often lack the resources they need to defend against these type of attacks.