Smoke is pulled up a chimney because of draft, which is a combination of pressure, speed and volume. The temperature of the gases contained in the fireplace smoke plays a factor as well.
Draft produces a vacuum effect inside the chimney. Because the air pressure inside a chimney is often less that the air pressure inside the house itself, smoke and air are naturally drawn upward. The main reason for the difference in air pressure is temperature. Since the air inside the chimney is warmer than the temperature of the air inside the rest of the house, a vacuum is formed.