A gravity bong creates the suction to milk the hit on it own, and then you just hit the smoked filled container. A classic example of a gravity bong is a two liter pop bottle with the bottom cut off of it and bowl where the lid is. You put the two liter down into a bucket of water until the water almost reaches the top of it, and then you slowly pull the two liter up while lighting the bowl and the suction created from the water leaving the two liter pulls the smoke into it, and after its milked up with smoke you take the bowl off and clear the bottle. For a normal bong you have to draw air through it to pull the smoke into the chamber, pull the slide, and then keep drawing air to clear it.
Thanks! They are a lot of fun, you can take some massive hits from a gravity bong. A few tips though, don't push the two liter into the water with the bowl loaded or it will puff out all the green, and try and hit it as soon as you can so the smoke doesn't get too stale.
I'm a huge fan of gravity bongs. Any time I'm low on bud or just want to make it last, I'll make one out of a two-liter soda bottle and a couple tiny bowls will get me nicely faded.
Regular bongs are the shit. They arent all messy and cumersome as gravities and waterfalls. A normal bong, you just load a hit, milk the shit out of it, then snap it. All done, ready for a basicly infinate number of hits.