What is transpirational pull?

In plants and trees, water is sucked up the stem from the
roots where it evaporates at the leaves. The upward movement of the water is accounted
for by a mechanism, called transpiration pull, that involves the evaporation of water
from the leaves. The suction force which pulls water into the leaves (by transpiration)
and up the xylem vessel is termed transpiration pull.

For a
better idea of water transport in plants, watch this
video:

http://www.youtube.com/watch?v=w6f2BiFiXiM&feature=player_embedded