The CHDK is a firmware add-on for Canon's compact digital cameras, based on DIGIC II and DIGIC III processors.
The CHDK implements new features like RAW, battery indicator, live histogram (RGB/luminance), zebra mode (blinking areas of over-/underexposure), DOF calculator, scripts (intervalometer, exposure/focus/... bracketing etc.), text reader, file browser, calendar, games and much more. There are now several different builds, implementing features assisting taking 3D-Stereo photos, motion detection etc.
The CHDK firmware add-on does not touch the original firmware of camera. It is an additional program which gets loaded into the memory of the cam.