Some more background info, courtesy of The Register:
Basically, this all goes back to the theft of a couple of spying tools developed by the US National Security Agency (NSA) - Eternalblue and Doublepulsar. The Wannacrypt ransomware worm currently doing the rounds is developed from Eternalblue. What's not getting as much publicity at the moment is that it's also installing Doublepulsar - a "backdoor" access tool that allowed the NSA - and presumably now other criminals - to remotely control infected machines. 
Final words from The Register:
If you haven't been infected, make sure your security patches are up to date. Kill off SMBv1 at the very least, and block access to it from outside your network. The exploits the malware uses have already been patched, and there's no excuse for getting caught out as a private user. It's understandable that IT managers with annoying corporate policies and heavy workloads have been forced to hold back patches, or are unable to apply them. If you can update your installations, drop everything and get patching.
And we'd sure appreciate it if you could stop clicking on attachments from unknown parties, too. ®