Via the always authoritative Wikipedia:

Sky burial (Tibetan: བྱ་གཏོར་, w bya gtor), lit. “alms for the birds”[1]) is a funerary practice in the Chinese provinces of Tibet, Qinghai, Sichuan andInner Mongolia and in Mongolia proper wherein a human corpse is incised in certain locations and placed on a mountaintop, exposing it to the elements (mahabhuta) and animals – especially predatory birds. The locations of preparation and sky burial are understood in the Vajrayanatraditions as charnel grounds.

The majority of Tibetans and many Mongolians adhere to Vajrayana Buddhism, which teaches the transmigration of spirits. There is no need to preserve the body, as it is now an empty vessel. Birds may eat it or nature may cause it to decompose. The function of the sky burial is simply to dispose of the remains in as generous a way as possible (the source of the practice’s Tibetan name). In much of Tibet and Qinghai, the ground is too hard and rocky to dig a grave, and, due to the scarcity of fuel and timber, sky burials were typically more practical than the traditional Buddhist practice of cremation. In the past, cremation was limited to high lamas and some other dignitaries,[2] but modern technology and difficulties with sky burial have led to its increasing use by commoners.[3]