Live boot CDs are CDs (and DVDs also) that you can put in your computer and boot up into an operating system from the CD. There are a several uses for live boot CDs not limited to training, demonstrations, data recovery, recovering an unbootable OS(Operating System), safe web-browsing, creating a network appliance, diagnostics, migrating an OS to a new drive or even "covert operations".
How they work
Modern operating systems often work best when they have a hard drive to use as temporary storage space as most programs need somewhere to store configuration files and user information. When you open a ZIP file for example, the data needs to be written to disk before you can read or use it. Live Boot CDs normally set up a simulated drive that uses part of your computer's RAM to store files. The operating system itself and programs generally load directly into memory from the CD. Sometimes a specialized 'low-memory' Live Boot version is required to be able to operate on systems with small amounts of memory (anything less than 256MB of RAM may have problems with some Live Boot OS's)
Live Boot Linux
While you could create your own Live Boot Linux disk, there are already a number of them ready to download and use. The most prominent would probably be Knoppix. Knoppix is designed to be a Live Boot enviroment, but it can be installed to your hard drive if you choose. There are currently two versions; a DVD version that almost totally fills a single-layer DVD and a 'lite' version that fits on a CD. Even the lite version is packed with web browsers, office applications, graphics tools, system tools and games. Some websites offer detailed instructions on how to take a prebuilt Knoppix distro(short for distribution - that is, one of the many flavours of Linux) and make your own customized version (some Linux knowledge is recommended). Other Live Boot OS's include Linspire(or the free version; Freespire), DSL and Morphix.
Live Boot Windows
If you want a Live Boot Windows CD, you will need to create one yourself. Don't worry, it's not as bad as you may think. Using Bart PE, you can take your existing Windows 2000 or Windows XP install disk and build your own Live Boot CD complete with tools and applications without having to know the inner workings of the process. There are a number of PE Plugins that allow you to add applications and tools to your PE (which stands for Pre-execution Environment I beleive) disk. Often, freeware tools and programs will be included with the plugin, while other programs that have license restrictions or that you need to buy will have only the plugin files and instructions of how to use the plugin. There are some good sites that have detailed instructions for how to use Bart PE. In my opinion, anyone that does regular maintenance on Windows computers should have a Windows PE disk.
If you surf the web from a Live Boot disk, you're practially immune to virus and spyware attacks. If something goes wrong, reboot and your system is clean. If you have already gotten a virus and can't boot your computer anymore, you could boot a Windows PE disk to run a virus scan and fix any damage done. Or if nothing else, copy your vital data to a USB drive (after cleaning the virus of course!). I've used a PE disk to migrate the entire OS from one hard drive to a new, larger drive without having to mess with imaging software. One of the key benefits of Live Boot Linux CDs is that people unfamiliar with Linux can give it a test drive without having to reinstall their computer or risk losing data. DSL Linux is only 50MB in size and is designed to be loaded into a USB flash drive as an alternate boot media (for those newer systems that support booting to a USB flash drive). In my previous article about emulators, I mentioned virtual machines. Most virtual machines can be configured to use a CD ISO image file instead of your real CDRom drive so you can use one of these to test your CD before you burn it to a disk.
If you work with computers, you should definately have at least one or two of these CDs in your toolbox. Happy booting!