The basic difference is the ELPH series are meant for the person who doesn't necessarily want to be bothered with anything but Automatic mode, while the SX series are for those who want more zoom, and even more camera controls. The GX series are for those who want the ultimate in control and high image quality.
The ELPH are also very pocketable cameras, while the other PowerShots are not all that small. The ELPH cameras can be used pretty much right out of the box with very little instruction, while the rest require some reading to get the most out of the cameras.
Hope this is helpful.
Steve M.
