What's a Live Cd/DVD you ask??? A live CD is a all in one bootable CD or DVD, containing various operating systems and tools. They are used to troubleshootng machines, removing viruses, doing computer forensics, trying out an OS before installing it, or as mobile gaming platform. The list goes on!!! There are many different Distro's out there mostly linux based, that you can download and try for free. In most cases you simply download the ISO file and burn it onto a CD or DVD. Once this is done you pop the disk in, reboot the machine, and away you go. The disk will bypass the OS on the hard drive and load the system into memory. I have several I use for various applications.
Some of my favorites are:
SUSE 9.3 Live DVD
For a complete list and many more distros for download go here.