Read more in the Shellshock FAQWhat is Shellshock?
Shellshock is a nickname for a bug in the Bash (Bourne Again SHell) command-line interpreter, also known as a shell. The Bash shell is widely distributed as the default command-line interpreter on many operating systems including most flavors of Linux, many flavors of Unix, some flavors of BSD, and Apple's OSX (since 10.3).
Pay attention to this one folks. Patch your UNIX and Linux boxes. Look out for those linux on windows environments like Cygwin.