The dedicated men and women of Transportation Alternatives (TA) describe congestion pricing as "the practice of charging motorists more to use a roadway, bridge or tunnel during periods of the heaviest use. Its purpose is to reduce automobile use during periods of peak congestion, thereby easing traffic and encouraging commuters to walk, bike or take mass transit as an alternative."
Wiki adds: "This variable pricing strategy regulates demand, making it possible to manage congestion without increasing supply. At the same time, users will be forced to pay for the negative externalities they create, making them conscious of the costs they impose upon each other when consuming during the peak demand, and more aware of their impact on the environment."
"Congestion pricing is the most powerful policy tool at the hands of City officials to reduce unnecessary driving, promote environmentally sound transportation, and finance 21st Century improvements to our aging transportation infrastructure," says TA. "Congestion charges have proven effective (and popular) in cities around the world."
More information is available from the US Department of Transportation.