Bongs that don't use sliders use a carb. A little hole in the back that you put your finger over to gather smoke into the bong, and then release it to go into your mouth. I like sliders better, you can get custom bowls and better downstems. Plus, most really expensive (and reeeally nice) bongs use sliders.

It's like a car. You can get an automatic or a manual transmission. Automatic's are easier, but manuals are cooler, and all of the best cars have manual transmissions.