Threat Spotlight: Trojan.
Trojan.Ransomlock is a detection for Trojan horse programs that lock the desktop of a compromised computer making it unusable.
The threat may arrive on the compromised computer by various means, such as visiting malicious sites, by opening untrusted links or advertisement banners, or by installing software from untrusted sources.
These programs attempt to convince the user to pay money in order to have their computer unlocked and use a variety of different techniques in order to encourage the user to pay the ransom.
More information on Trojan.Ransomlock is available in the threat family writeup.