The beauty of a Live CD is that it gives you a chance to access your computer or a batch of alternate applications without actually having to load up your operating system.  You only need to pop the CD into your optical drive and boot it up from your BIOS -- this self-contained environment runs independent of anything that's located on your drive partitions, even though you can still perform a variety of tasks that manipulate the data on your drives.  

For example, you can test our new Linux distributions using a Live CD, saving you the time and hassle of blanking an entire partition just to see if it's the right distribution for you.  You can also manipulate the partitions of your drives using a Live CD, expanding and creating volumes to create alternate locations for new operating systems, files, or whatever it is you'd use a separate volume for.  Live CDs are great for troubleshooting your system (or saving your data) when your primary operating system won't boot, and they can also be used to break through Windows installations that you've lost the password for.

All that functionality...  and you don't even have to install a single program on your machine!  Click the link to check out some of the best Live CDs that you should have sitting on your desk.