Tibetan prayer wheels contain mantras written on strips of paper. Spinning the prayer wheel is like saying the mantra repeatedly. Traditional prayer wheels contain hundreds, thousands, even millions of copies of the mantra "Om mani padme hum." evoking Chenrezig, the Bodhisattva of compassion (Avalokitesvara).
Smaller prayer wheels are turned by hand. Others are powered by water wheels, hot air raising from candles, or the wind. Prayer wheels are spun clockwise when looking down on them.
Turning a prayer wheel is like chanting the mantra "Om mani padme hum" (Jewel in the Lotus of the Heart). Reciting this powerful mantra builds positive karma by evoking the Buddha of Compassion.
The Sakya Monestary here in Seattle make prayer wheels containing 1.3 trillion (with a "t") mantras and sutras on a set of DVD’s. Read more here. I visit this monastery to turn the wheels outside fairly regularly.