What is 'WanaCrypt0r 2.0' ransomware and why is it attacking the NHS?

Malicious software has attacked computers across the NHS and companies in Spain, Russia, the Ukraine and Taiwan. What is it and how is it holding data to ransom?

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'WanaCrypt0r 2.0' malicious software has hit the NHS, some of Spain's largest companies including Telefonica, as well as computers across Russia, the Ukraine and Taiwan, leading to PCs and data being locked up and held for ransom.

The ransomware uses a vulnerability first revealed to the public as part of a leaked stash of NSA-related documents in order to infect Windows PCs and encrypt their contents, before demanding payments of hundreds of dollars for the key to decrypt files.