A diffuser is another type of downstem for a bong. Contrary to a downstem, a diffuser resembles a test tube as its sealed on the end or pierced with many small holes or slits. These slits break up the smoke into smaller bubbles than a normal downstem.
Why the need for smaller bubbles? The smaller the bubbles, the greater the surface area of the smoke that contacts the water, thus filtering and cooling the smoke more efficiently. This makes the bong almost silent silent. Many glass companies sell diffusers to fit their bong: Roor, Phx, and Bluedot.