The Venturi effect is a phenomenon which is described by Bernoulli's equation. When a fluid (air, water, sewage - anything that flows) is flowing through a pipe, and is forced through a narrower section of pipe, the pressure decreases while the velocity increases. (Sort of like the Bernoulli effect for enclosed conduits.)

As it turns out, if you punch holes in said pipe at the "mouth" (the wider intake area) and the "throat" (the narrower area), and run a tube between them, the Venturi effect will suck air from the mouth end of the tube towards the throat end of the tube. Indeed, the tube need only be connected at the throat part; depending on the pressure of the fluid traversing the narrow bit of pipe, whatever is on the other end of the tube will be subjected to some suction.

Uses for the Venturi effect:

• Aquariums. Most power heads include adjustable Venturi devices, usually a tube that connects at the water outlet and has an adjustable air valve at the other end. The Venturi effect causes air to be sucked in through a short hose and intermixed with the water being shot out by the pump. This aerates the water.
• Carburetors. Commonly, air is forced through a narrowing pipe inside a carburetor. At the narrow end, one or more fuel ports are drilled. The Venturi effect helps to suck the fuel in and atomize it with the air before it is distributed to the engine's cylinders.
• Venturi meters. As previously mentioned, a hose connected between the "mouth" and "throat" sections of the pipe will result in air being sucked through the hose from the mouth to the throat. It turns out that if you orient the hose to hang from the bottom of both sections of pipe, and fill it with a specific amount of fluid (heaver than the kind the pipe is transporting of course), that the throat section of the pipe will draw the heaver fluid up towards itself. Since fluid seeks its own level due to gravity, the heaver fluid will try to equalize itself such that it is at the same level on both sides of the hose. When the Venturi effect starts to pull up on the heavier fluid, it will rise somewhat, and the fluid on the mouth end of the tube will drop somewhat. The difference in height can be plugged into a formula and used to determine the velocity of the fluid in the pipe, since at a higher velocity it will draw the heavier fluid closer and closer to the throat.