A prayer wheel is a cylindrical 'wheel' on a spindle made from metal, wood, stone, leather, or even coarse cotton. Traditionally, the mantra Om Mani Padme Hum is written in Sanskrit externally on 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.
The earliest recorded prayer wheels was written by a Chinese pilgrim around 400 CE. in Ladakh. The concept of the prayer wheel is a physical manifestation of the phrase "turning the wheel of Dharma," which describes the way in which the Buddha taught.
According to the lineage texts on prayer wheels, prayer wheels are used to accumulate wisdom and merit (good karma) and to purify negativities (bad karma). In Buddhism, Buddhas and Bodhisattvas have created a variety of skillful means to help bring practitioners ever closer to realizing enlightenment. The idea of spinning mantras relates to numerous Tantric practices where the Tantric practitioner visualizes mantras revolving around the nadis and especially around the meridian chakras such as the heart and crown. Therefore prayer wheels are a visual aid for developing one's capacity for these types of Tantric visualizations