If somebody/something is efficient, he/she/it works in a well-organized way without wasting time or energy.
He’s not very efficient; he keeps filing letters in the wrong place, he works very slowly, and he keeps forgetting things.
The postal service is even less efficient than the telephone system.
If something is effective, it has the right effect: it solves a problem or gets a result.
My headache’s much better. Those tablets really are effective.
I think a wide black belt would look very effective with that dress.
Michael Swan, Practical English Usage, Third Edition, Oxford University Press, 2006.