A zero setback seat posts is completely straight, with the mounting mechanism at the top. A set-back seatpost is shaped sort of like a reversed number "7" or rather an inverted letter "L". The mounting mechanism is "set back," typically about 25mm (but they vary) or from the actual seatpost thus extending the reach, and also changing the angle of your leg while pedaling.

A picture says 1,000 words - there are photos at this website

http://en.wikipedia.org/wiki/Seatpost