A bootable CD-ROM or DVD-ROM that contains the original installation of the OS and applications for a particular computer. When booted, a recovery program (recovery console) returns the computer to its initial state. Most recovery programs wipe out the entire hard drive, while some retain the user's data.
In lieu of an optical recovery disc, vendors often store the recovery image on a separate partition on the hard drive. If the operating system has become corrupted, but the drive still works, running the recovery program from the hard disk will restore the machine to its initial state.
The Better Approach
Far more advantageous than performing an optical or hard disk recovery to the computer's original state is to periodically back up the entire primary hard drive onto a second internal hard drive or to an external drive. See backup software