The Apple iPhone 3G dock is just like its name suggests; it is a docking station specifically made for the iPhone 3G.
As strange as it may seem, Apple has decided not to include a dock with its new iPhone 3G; you'll have to use a regular cable to sync it. The old iPhone dock isn't quite compatible with the 3G version, because of the iPhone 3G's new contoured shape. So if you want a dock for your iPhone 3G, you'll have to get this Apple iPhone 3G dock, with a special contoured back made to fit the iPhone 3G. You can use the dock to charge and sync your iPhone 3G as well as connect it to a stereo or speakers to play music.