This is a very succinct and thought-provoking article which explores the origin and relationship between the Pawo (Shaman), Ngakpa (Magician) and Lama roles, outlining traditional roles with particular reference to the ancient Bonpos:

A couple of other good resources:

An interview with Khetsun Sangpo Rinpoche about the role of the Ngakpa and the role of the shaman, the differences and similarities: