Describe yourself

Yesterday i just thought of a question, like I'm always asked at interviews:

" Tell me about yourself." "Can you describe yourself?"

Honestly I hate to be asked this question. Duno y. Guess I dint know about myself? Can I answer "You'll know me as time goes by. So dun ask me this now." -_-! Then the next thing I know will be out of the company without any successful job.

There's always two sides to each of us. If you say you are cheerful, optimistic, able to take stress and blah blah. Then what if the first task only, you are not able to do all these?

Any idea how you would answer if people ask you this?