Magic Lantern is a great software and this amazing add-on completely changed the video quality and the possibilities of many Canon's DSLR cameras. If you wonder how Magic Lantern works, you should really watch this video. It features Georg Hofstetter and Michael Zoller who talk about the firmware and presents a live demonstration of Magic Lantern, showing some of its unique functionalities and covers the hardware and software internals of Canon’s camera operating system DryOS. From the video you will also learn that programming your Canon camera is quite similar to programming any embedded ARM device in plain C. If you make the decision to mod your camera you should be aware of some legal aspects, but this topic is also discussed in the video.
"Magic Lantern is a Free Software add-on for Canon EOS DSLR cameras, that offers increased functionality aimed mainly at DSLR enthusiasts and power users. It is very similar to CHDK for Canon compact cameras and it runs alongside Canon’s own firmware, by hooking into the startup process."