Evap sucks ass and everyone hates it, but from gen II on, it's actually pretty easy to diagnose. gen I is the worst and whoever designed it belongs in prison somewhere. choose which system you're working on to the right.
evaporative emissions systems or evap systems deal with the pollution caused when raw fuel evaporates. back in the day, you used to be able to just pour your gas into a gas tank. over time, as it was exposed to the atmosphere, it would slowly evaporate, causing drivers to lose fuel and causing the chemicals in the gas to escape into the atmosphere.
over time, people realized the only way to deal with evaporating fuel was to trap it. fuel tanks were hermetically sealed and charcoal canisters were added. the canister's job was to trap the fuel vapor as it was evaporated. then a purge valve would open now and then and use engine vacuum to suck unburned fuel vapor into the intake to get burned like normal in the engine. in order to refuel the tank, a vent valve was also added, which allowed air to be drawn into the fuel tank as the fuel level dropped. without this vent valve, either the vacuum in the fuel tank caused by dropping fuel levels would be high enough to cut off the fuel flow completely, or the tank would just collapse under the power of the vacuum that was created.
as a result, evap systems have three basic components, a purge valve, vent valve, and canister to go with the fuel tank and gas cap. many systems have additional components, but they all serve to help support the three basic ones.