It collects fuel vapour and sends it back to your intake, an emissions thing. Once removed it doesn't seem to have any effect on the engine, just DON'T block off the hose that goes from the tank to the canister, vent it to atmosphere down into the inner wing. Blocking it off can cause excess back pressure in the fuel system and things will start blowing off.
The official answer is:
An evaporative canister is part of the evaporative system that is within the emission system. Hydrocarbons are a form of pollution that gasoline releases as it evaporates or after is is burned. While fuel is stored inside the tank, fumes build up and are retained inside this canister. The canister contains charcoal that absorbs the hydrocarbons. When the engine is started, a valve opens that allows fresh air to come in and purge the fumes (hydrocarbons) into the air intake and then into the combustion chamber for burning. I understand that more hydrocarbons are released into the atmosphere by leaving your fuel cap off, than what comes out of your tailpipe while the engine runs. If there is a leak anywhere in the evaporative system, such as a bad hose, canister leak or leaving the fuel cap loose, the check engine light will set.