Do not say what you don't know. Do answer concisely and to the point. If open ended go into relevant details. If personal you know yourself better than anyone. Agree with almost all previous replies, the more you know about the company, their customers, the position being seek/offered, their benefits, their market share... the better you can answer. Above all, if you are the type that get nervous, try to get a solid sleep and avoid stimulants (coffee,tea...) and if face to face, look straight at the person asking. Don't sweat it, you probably know more than you "think" you know. :)