A Tibetan prayer wheel, also called a mani wheel, is a device designed for praying as efficiently as possible. The prayer "Om Mani Padme Hum" is printed onto slips of paper that are used to fill a metal container that fits around an axle, around which it can spin.

Spun Clockwise

Tibetan prayer wheels are always spun clockwise. This way, the words go past the viewer in the same order as they would if being read. The direction also echoes the rise and fall of the sun.

Small to Large

Mani wheels can vary in size from small handheld prayer wheels to significantly larger structures capable of holding vast quantities of prayers. Some of these wheels are turned by teams of people, while others employ waterfalls, river or wind.

Om Mani Padme Hum

The Tibetan prayer, or mantra, Om Mani Padme Hum is believed to attract the attention of the god of compassion. Written versions of the prayer, which can be read or spun in the prayer wheel, have the same effect.

Enlightened Sound

Each syllable has a unique meaning. Om prays to achieve generosity. Ma prays to achieve ethics. Ni prays to achieve tolerance and patience. Pad prays to achieve perseverance. Me prays to achieve concentration. Hum prays to achiece wisdom.

Computerized Prayer Wheels

The Dalai Lama has declared that having a prayer wheel on your computer counts in the same way a more traditional mani wheel would. Each spin of the hard disk can represent a mantra, and the disk can hold multiple copies of the warrant, making this one of the most efficient ways to pray.

Source:

Dharma-Haven.org: The Prayer Wheel: Spiritual Technology from Tibet

More Information:

Dharma-Haven.org: Advice on the Benefits of Prayer Wheels