A short, short version because this seems to be hitting so many news sites, and of course when that happens some get it wrong.
Basically this is a very large attack campaign delivering ransomware. The initial fee was $300 but soon changed to $600 -- I haven't read of further price increases but it's of course possible. [At the same time, per Threatpost: "A new malware family called Jaff has been identified by researchers who say they are currently tracking multiple massive spam campaigns distributing the malware via the Necurs botnet." It's asking for ~$3,000.]
Two things make WannaCry newsworthy... One, it reportedly near shut down the UK's National Health Service, along with victims in a reported 99 countries. Two, rather than use the much more common methods of infection, e.g. spam, it uses one of the alleged NSA tools released by the Shadow Brokers. And yes, that vulnerability was patched 2 months ago via Windows Updates.
If nothing else this ransomware's success hints at the large numbers of people and businesses who do not update Windows. While it'll be a while before postmortems are published, WannaCry also seems wormable, meaning it just needs to get one foot in the door so-to-speak, infect one device on a network. Probably most security software also blocks WannaCry by now.
