The question seems an easy one to answer but mostly it has been seen that when this question is put before interviewee he/she is not very clear about their strengths and weaknesses.
Strength should define your skills. Your skills can be knowledge based or your personal attributes like punctuality, honesty, good communication skill, judgment power, leadership skill, hardworking, friendly, etc.
Weaknesses is the most dreaded part of the question. Everyone possess some weaknesses, but who wants to admit to them. The best way to handle this question is to select a trait and come up with a solution to overcome your weakness. Then let the interviewer know that although you may not be perfect, you are working on any shortcomings you have.