Live CDs/DVDs are CDs/DVDs that boot an operating system.  They use what's called a RAM disk to get read/write operations to happen.  Many operating systems (Mac OS X, Windows Vista/Server 2008, Linux) also use the concept of Live CDs to install an operating system to the hard disk, but it's called a PreOS environment. Live CDs are mostly used to test an OS before you install it to the HD, repair an OS on the hard disk, or to boot an OS easier instead of having to go through an entire install process.  The original Xbox had a Live CD you could use to install one of the custom dashboards.  Live CDs are portable, so you can burn it to a disk and go from one PC to the next with it.

Live CDs are downloaded as a disk image so you need something like <a href="http://www.alcohol-soft.com/">Alcohol 120%</a> to burn it to a disk.

Here's are lists of Live CDs that boot a distribution of Linux or BSD.  There are several hundred of these with many different purposes:

http://www.frozentech.com/content/livecd.php

http://www.livedistro.org/node/list

If you want to build your own custom linux live CD, you can go here:

http://custom.nimblex.net/

Windows has Live CDs as well.  One of them is BartPE, which allows you to customize a Windows XP live CD with tools you need to repair your computer.  BartPE is based on WinPE, which is Microsoft's product that boots Windows off a CD/DVD for the only purpose of installing Windows to the HD.  BartPE is free:

http://www.nu2.nu/pebuilder/

Then there's Winternals ERD Commander, which isn't free but comes with a bunch of tools to repair your computer.  Winternals was bought out by Microsoft so it's now called &quot;Microsoft Diagnostics and Recovery Toolset&quot;:

http://www.microsoft.com/windows/products/windowsvista/enterprise/dart.mspx

You can download a trial here:

http://www.microsoft.com/downloads/details.aspx?FamilyID=5D600369-0554-4595-8AB4-C34B2860E087&amp;displaylang=en

Here's a video of Microsoft Diagnostics and Recovery Toolset:

http://www.microsoft.com/windows/products/windowsvista/editions/enterprise/videos/drt.asx