This security hole has the potential to be the most dangerous bug of the last few years. Everybody should change the passwords for internet services they use — once the service providers have fixed the vulnerability in their systems.
Why is the bug so dangerous? Because it subverts the basic encryption technology used by two thirds of the websites and services on the internet.
This article by Jeff Goldberg at AgileBits explains the HOWs and WHYs and what to do to protect yourself and your data.
For a more succinct explanation of how the bug works, refer to this comic by Randall Munroe of XKCD fame.
Disclaimer: I work for AgileBits.