If somebody or something is efficient, he/she/it works in well-organized way without wasting time or energy.

Ex1: He’s not very efficient: He keeps filing letters in the wrong place, he works very slowly, and he keeps forgetting things.

Ex2: 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.

Ex1: My headache’s much better. Those tablets really are effective.

Ex2: I think a wide black belt would look very effective with that dress.