CHDK is a firmware enhancement that operates on a number of Canon Cameras. CHDK gets loaded into your camera's memory upon bootup (either manually or automatically). It provides additional functionality beyond that currently provided by the native camera firmware.

What extra capabilities does CHDK provide?
The current set of extra capabilities fall into six categories:
a. Enhanced ways of recording images – you can capture still pictures in RAW format (as well as JPEG), and for video images you can have increased recording time and length (1 hour or 2 GB), and a greatly increased range of compression options.
b. Additional data displays on the LCD screen – histogram, battery life indicator, depth of field, and many more.
c. Additional photographic settings that are not available on the camera by itself – longer exposure times (up to 65 seconds), faster shutter speeds (1/25,000 sec, and faster in some cases), automatic bracketing of exposure, etc.
d. The ability for the camera to run programs ('scripts', written in a micro-version of the BASIC language) stored on the memory card – these programs allow you to set the camera to perform a sequence of operations under the control of the program. For example, a camera can be programmed to take multiple pictures for focus bracketing, or take a picture when it detects that something in the field of view moves or changes brightness.
e. The ability to take a picture, or start a program on the memory card, by sending a signal into the USB port – you can use the USB cable to take a picture remotely.
f. The ability to do a number of other more useful (and fun) things, such as act as a mini file browser for the memory card, let you play games on the LCD screen, etc