What has Edward Snowden taught us about Quantum Cryptology?

The latest story to hit the headlines as a result of the Edward Snowden leaks is that the US National Security Agency (NSA) has a $79m (£48m) research project called 'Penetrating Hard Targets' which in part wants to construct a "cryptologically useful quantum computer" which would break all forms of public-key encryption known to man. Which begs the question what are quantum computers and quantum cryptography, and how worried should we be about them?