A Live CD is capable of booting an Entire Operating System directly from a CD or DVD, using only RAM, not touching the Hard Drive's contents. As such, they're perfect for System Repair, Problem Diagnosis, or just the convenience of running an OS on a PC without needing to worry about security risks, like Stored Passwords.
Microsoft Windows and ReactOS both have Live CD's on offer, the former having to use a PE tool to obtain a working copy. The Live CD is also a popular way for users to try a Linux Distribution. Gentoo, Ubuntu, and Knoppix (Gnoppix, too), can be used via Live CD, although this method is much, much slower than directly loading from a Hard Disk, owing to simple speed limitations when accessing External Optical Media, compared to Internet Magnetic Media.