Have a quick lookee here: http://www.everyonedoesit.com/online_headshop/

There's quite a big difference in the three - a bong is just a massive tube device with some water in the bottom to help cool and filter the smoke. Lets you have MASSIVE hits, sometimes even being able to cane an 1/8th in a matter of seconds depending on how your throat can handle smoke. As with hookahs and waterpipes, they come in many different shapes and sizes (namely large, extra large and the 'God' size bong).
Hookahs are... er... I can't really describe these very well. They're kinda like bongs, but with big hoses attached. Sometimes they come with multiple hoses for parties and such. The sizes and designs vary greatly.
Waterpipes are just like regular pipes, but hold water to cool the smoke, so it's sorta like a mini-bong.