Ransonware (often referred to as Cryptolocker) is a malicious virus threat in today’s environment.
If the virus gets onto your computer, it will begin locking down files on your computer by writing a hidden secured password to those files. It will then display a message that you have XX number of hours or days to pay the ransom to get the password to unlock your files.
Unless you have solid backup, your two options are: pay the ransom or lose all your files. The ransom could be in the hundreds of dollars. For a California hospital, that ransom was to the tune of $17,000 dollars! 
MalwareBytes has a blog post on their site dated March 1, 2016 which gives an in-depth analysis of how such a ransomware virus works.
I found it an interesting read, and thought I would pass it along.
You can go directly to the blog post and read it, or I saved it as a PDF file that you might find easier to read.
Kudos to MalwareBytes!