LiveCD
From Knoppix Documentation Wiki
En español: CDVivo.
A LiveCD is an operating system stored in a CD-ROM that can be executed from it, without installation in the hard drive, although it can be optionally installed in the primary or secondary hard drives.
One can use a ready CD or burn an ISO image downloaded from the Internet, using a download manager.
The most popular are the GNU/Linux LiveCDs, specially Knoppix (based in KDE) and Gnoppix (based in GNU Gnome.
Knoppix can also be run from a USB pen, [1] explains how.