What is the 'ransomware' WannaCry worm?

Malicious software called “ransomware” has forced British hospitals to turn away patients and affected Spanish companies as part of a global outbreak that has affected tens of thousands of computers.
 
How does it work?
 
WannaCry is a form of “ransomware” that locks up the files on your computer and encrypts them in a way that you cannot access them anymore.
 

How does it spread?
 
Ransomware is a program that gets into your computer, either by clicking on the wrong thing or downloading the wrong thing, and then it holds something you need to ransom.
 
In the case of WannaCry, the program encrypts your files and demands payment in bitcoin in order to regain access. Security experts warn there is no guarantee that access will be granted after payment.

 
What is so special about wannacry?
 
WannaCry is not just a ransomware program, it’s also a worm. This means that it gets into your computer and looks for other computers to try and spread itself as far and wide as possible.
 
Ransomware has a habit of mutating and so it changes over time in order to find different ways to access computers or to get around patches. Many security firms are already aware of WannaCry in past forms and most are looking at this one right now to see how it might be stopped.