A device which cools and filters smoke. Usually used when smoking marijuana, but it could be used for any smokable material whose drug does not dissolve into water. It consists of a bowl, to put the plant matter in, connected to a longish tube whose end is submerged in water. (Actually, any liquid will do. It's just silly to use anything other than water, really...) The water container usually tapers up to a mouth-sized opening at the top, where the user sucks while keeping the plant matter lit. The smoke travels down the tube, bubbles through the water, and fills the chamber of the bong. With larger bongs, you may need to do one inhalation to fill the chamber with smoke, then a second to actually get any smoke.

Somewhat counterintuitively, hot water "cleans" smoke much better than cold water does. Cold smoke is far easier to inhale, though. One solution is a two-chambered bong:

      ___    suck here
bowl  |  | |   |
   ( )| \\ |   |
    \\|  \\|   |
     \\  |\\   |
      \\~| \\~~|
      |\\| |\\ |
      |__| |___|
      HOT   COLD

Where the really nasty stuff comes out in the first, hot chamber, then gets sucked into the cold chamber where the smoke is cooled down before inhalation. This is a lot of trouble though...