From Strategy Page:
The incident began with the activation of ransomware malware called WannaCry. What made WannaCry so dangerous was that it made use of several capabilities including a hidden (but findable) backdoor program that tried to spread WannaCry to Microsoft Windows computers that had a known vulnerability but were not updated to remove the vulnerability. This automatic spread of malware is called a worm…
Source: Information Warfare: An Endless Mystery Called WannaCry
Very interesting including speculation that the attack came from North Korea, but then again maybe not. Plus a useful glossary of computer terms.