The Raspberry Pi is tiny $35 computer with a 700 MHz ARM11 processor and 256MB of RAM. While it won't run Windows, it can run a variety of Linux-based operating systems including Debian, Fedora, and even Raspbmc — an operating system designed around the XBMC Media Center application.
But the Raspberry Pi's ARM-based processor isn't exactly a speed demon, and while those operating systems all run on the computer, they don't necessarily run very quickly.
Raspbian, on the other hand, is an upcoming operating system that looks like it could run circles around the others.
While earlier operating systems released for the Raspberry Pi were basically ported from existing versions, Raspbian is optimized specifically for the tiny computer's hardware.