Debian is an operating system that is based on GNU/Linux. It was previously recommended by the Raspberry Pi Foundation as the standard distribution, but Raspbian is now being recommended.
The Raspberry Pi Foundation provides a Debian Squeeze image, recommended for use with the Raspberry Pi. This image includes Python, gcc, and a range of example applications for programming and teaching.
For information regarding installation, as well as up-to-date images, please visit the Raspberry Pi Quick Start Guide.