Knoppix

Knoppix is a bootable CD. Knoppix is also referred to as a liveCD. The CD contains a collection of GNU/Linux software (Debian Linux with LXDE desktop), automatic hardware detection, and support for many graphics cards, sound cards, SCSI and USB devices and other peripherals.

Knoppix also exist as liveDVD. It will NOT fit on a FATxx filesystem so you need ext3 or some such to copy the DVD .iso to hard drive. You can use Windows to put it on NTFS for a Poor Mans Install to free up your DVD drive.

Knoppix can be used as a Linux demo, educational CD, rescue system, or adapted and used as a platform for commercial software product demos. It is not necessary to install anything on a hard disk. Due to on-the-fly decompression, the CD can have up to 2 GB of executable software installed on it.

A more detailed definition can be found at the Wikipedia article on Knoppix