What is the Shellshock vulnerability in Bash?
As of this writing (September 25th, 2014), the situation with Shellshock is changing so rapidly that you're probably better off using your preferred search engine instead of this FAQ. For example, you could search a news site for recent items which contain the word "Shellshock". For information about specific vulnerabilities related to Shellshock, you may find better results by searching for terms such as "CVE-2014-6271", "CVE-2014-7169", "CVE-2014-7186", or "CVE-2014-7187".
After things stabilize a bit, this FAQ page should be updated with a handy summary. In the meantime, here are a few links that should help you get started:
Search Google News for 'Shellshock bash', limited to the last 24 hours
Summary article from RedHat on how to determine if a system is vulnerable
Official US-CERT page on CVE-2014-6271, the first vulnerability in the series to be discovered
"Everything you need to know about the Shellshock Bash bug", by Troy Hunt