Traditionally, the mantra Om Mani Padme Hum is written in Sanskrit on the outside of the wheel. Also sometimes depicted are Dakinis, Protectors and very often the 8 auspicious symbols Ashtamangala. According to the Tibetan Buddhist tradition based on the lineage texts regarding prayer wheels, spinning such a wheel will have much the same meritorious effect as orally reciting the prayers. – Wikipedia.com
Every time I visit a monastery, I am struck by a few things: the cleanliness and aura of peacefulness; the gorgeous, brightly colored tangka paintings and murals adorning the walls; the larger-than-life statutes of the various avatars of Buddha, and the prayer wheel outside most monasteries.
Every time I walk around a monastery, running my hand along the prayer wheels, I feel a sense of peace and calm descend over me. Once, I even managed to work through a particular problem that had been plaguing me since a while. Monasteries, along with churches, often have this effect on me.