See LiveCD.

A CD-ROM that contains a working copy of an operating system or other application that is run without installing it. Used for demonstration and evaluation as well as system recovery, the software runs intact from the CD. If used for testing software, installation may be an option, but it is not the default. LiveCD functions can also be stored on a DVD or USB drive (LiveDVD or LiveUSB).

For Testing and Evaluating
LiveCDs and LiveDVDs are a popular way to test machine compatibility with a different operating system without installing it. For example, some versions of Linux come on bootable CDs and DVDs for testing hardware compatibility. Linux runs directly from the CD or DVD and is not installed unless the user chooses that option (see LiveDistro).

After years of dealing with software conflicts, many people are averse to installing a new application. However, software on a LiveCD or LiveDVD, which does not alter any files in the user's computer, is a way of trying something new before installing it. For a list of LiveCD software, visit www.livecdlist.com.

Restoring a Failed Computer
LiveCDs are widely used for troubleshooting a PC that does not boot, and most personal computers are configured to look for a bootable program on the CD or DVD before loading the OS from the hard disk. See bootable disk.