Transpiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stoma that are bordered by guard cells. Collectively the structures are called stomata. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.
The rate of transpiration is directly related to the degree of stomatal opening, and to the evaporative demand of the atmosphere surrounding the leaf. The amount of water lost by a plant depends on its size, along with the surrounding light intensity, temperature, humidity, and wind speed (all of which influence evaporative demand). Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate.
A fully grown tree may lose several hundred gallons (a few cubic meters) of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for this process.
For more, visit the Wikipedia entry for transpiration, URL: http://en.wikipedia.org/wiki/Transpiration