Live CD

A live CD, live DVD, or live disc is a complete bootable computer operating system which runs in the computer's memory, rather than loading from the hard disk drive. It allows users to experience and evaluate an operating system without installing it or making any changes to the existing operating system on the computer. Live CDs are unique because they can run on a computer lacking secondary storage, such as a hard disk drive, or with a corrupted hard disk drive or file system, allowing users to rescue data.

Live USB flash drives are similar to live CDs, but often have the added functionality of automatically and transparently writing changes back to their bootable medium. Also, the solid-state device pure electronic operation gives a significant speed advantage by eliminating the CD reader's intrinsic mechanical latencies. Write-locked Live SD WORM systems are the direct solid-state counterpart to live CDs and can be booted natively in a media card slot or by using a USB adapter. Write-locked Live SD systems avoid excessive write cycles or corruption by ill-conditioned software, like viruses, which can prematurely "ruin" a live USB drive's installation.

While a live CD typically does not alter any operating system or files already installed on a computer's secondary storage (such as hard disk drives), many live CDs include optional software mechanisms and utilities for altering the host computer's data stores, including installation of an operating system. This is important for the system management aspect of live CDs, which can be useful for removing malware, for drive imaging, and for system recovery.

The default option, however, is to allow the user to return the computer to its previous state when the live CD is ejected and the computer is rebooted. It is able to run without permanent installation by placing the files that typically would be stored on a hard drive into RAM, typically in a RAM disk, though this does cut down on the RAM available to applications.