The purge canister is a charcoal filled container which the fuel tank vents through, to reduce the vapour pressure within the tank. Akin to a scrubber in a hose, the canister traps the fuel vapour but allows the flow of air to equalize tank pressure with ambient.
The canister is also attatched to the (sub Patm) part of the engines intake system, usually the manifold. Whilst the engine is running the canister is emptied of trapped hydrocarbons (purged) with these being introduced into and combusted by the engine, along with the fuel.
Anymore questions, let me know...
MS