Prayer wheels are very common in the Himalayas. They come in all shapes and sizes. Some are hand held, some are mounted along spiritual sites such as stūpas. They can be very small, or big as a telephone booth (or even bigger), made of wood or metal.
A mantra, usually oṃ maṇi padme hūṃ (ཨོཾ་མ་ཎི་པདྨེ་ཧཱུཾ་ – the mantra of Avalokiteśvara – Buddha of Compassion) is depicted on the outside, either painted or embossed, depending on the material of the ma ni chos ‘khor – མ་ཎི་ཆོས་འཁོར་
Inside, around the central column – or the life tree, many more mantras are wrapped, which can, depending on the size of the wheel be anywhere between a few hundred to a few million mantras.
The turning of the prayer wheel is seen as just as beneficial as reciting the mantra.