There are many types of prayer wheels including large water-powered wheels, and those that can be held in one's hand.  Hand held prayer wheels are spun clockwise with the rotation of the wrist aided by a weighted chain used to increase momentum.  They are constructed of a cylindrical body of metal or wood mounted on handle.  Most Monk Sanghas, or groups, have their own styles and designs.

There is either an embossed mantra on the outside of the body, or written on the side of the body.  The most common mantra found on prayer wheels is "O Mani Padmi Om" which means "Hail, the Jewel in the Lotus"; however, they could be any of a great selection of pre-written mantras, or of personal origin.

The purpose of a prayer wheel is to set the mantra in motion.  This relaxing motion can be very useful in aiding the process meditation.  Since speaking can break the meditation mindset, prayer wheels can be used instead of reciting a mantra.

-Berig