Live CDs, DVDs and live discs are CDs and DVDs that contain bootable computer operating systems on them. Live CDs are distinct in that they possess the ability to run a complete and modern operating system on any computer that lacks a form of mutable secondary storage, like a hard disk drive for example. Live USB drives are somewhat similar in comparison to live CDs, though they often have added functionality in that they can automatically and transparently write changes back to the bootable medium. The “live” term is derived from the idea that a complete, operational and functioning operating system is contained on the CD.
Many of these Live CDs have mechanisms included that allow them to alter the data stores for the host computer, including the installation of a new operating system. This can be an important part of the system management aspect for live CDs, such as system recovery, drive imaging and the removal of malware. The default setting, however, is simply to allow a computer to be returned to its previous state as soon as the live CD has been ejected and the computer has been rebooted. It can run without any permanent installation because it places files typically stored on the hard drive into the RAM, typically in the form of a RAM disk, but this can cut down on the amount of RAM that is available to other applications.
Some Linux live CDs are specifically designed to test drive or demo an operating system, especially open source operating systems because you can acquire the CD and test drive the operating system without losing your current operating system or the data on your computer. Beyond this, live CDs are created for a wide variety of different purposes and to meet a wide variety of different needs.