Evaporative Emission Control: Also known as the EVAP system. Common starting in 1972.
Gasoline is always evaporating! The idea behind this system is to catch some of those vapors when they escape from your fuel tank. Early systems such as those found in smog-era cars catch only part of the evaporating fuel, but later systems incorporate a better fuel cap that completely seals the system.
The main component of the system is the vapor canister. It contains a replaceable charcoal-impregnated filter element to let outside air in, while keeping fuel vapors inside. The canister is round or squared in shape and is mounted near the radiator. This location keeps gas vapors safely away from ignition and exhaust components. This canister collects vapors that evaporate from your tank though a network of hoses and valves.
Once inside, the vapors wait to be pulled into the engine for a proper burning. The EVAP system keeps hydrocarbons from escaping into the outside air. Hydrocarbons are raw fuel and are obviously not good to breathe in.
The horsepower and weight penalty of this system is negligible. In theory, you might even get better fuel mileage by keeping the system in your car.