You have heard about Shellshock Bash bug, right? It affects the machines running Linux and / or Mac OS X (including your PC's, web servers, your smartphones, some of your devices at home) by making them vulnerable via the Unix command shell Bash, which happens to be one of the most common applications in those operating systems. The shell or command prompt is a piece of software that allows a computer to interact with the humans by interpreting text typed via the keyboard. A malicious user can get access to your input and inject code to the system, taking control or damaging it and its data.
This bug was baptized Shellshock by Security Researchers and you can find more information about it over here. This issue is not critical, but it can create severe problems. Keep yourself informed and watch for updates... then updated your computer / server / smatphone and other devices as quickly as you can.
Let's start with your computer. If you have a Mac OS X or Linux system, open the Terminal and run this line of code:
env x='() { :;}; echo Shellshock Bash makes you vulnerable' bash -c 'echo this is a test'
If you see the text "Shellshock Bash makes you vulnerable" as an answer, your system is, well... vulnerable. Three of the most popular Linux distributions, Fedora, Red Hat and Ubuntu already have patches available, probably Apple will soon release its fix.