If your email outlining a plan to reduce office paper waste resulted in a 20 percent reduction in paper use, that means your plan was effective: it did what you wanted it to do.
The adjective effective has many shades of meaning, but you'll usually see it describing something that's able to produce a desired goal. Effective traces back to the Latin word effectivus, from efficere, meaning “work out, accomplish,” and that meaning still holds true. Something effective gets the job done. As President, hopefully your policies are effective. We also use effective for something real and in effect, like the effective price of gas, or your effective grounding.