Its basic premise is the existence of an otherworld inhabited by more or less autonomous entities such as spirits, angels, demons, gods etc. The shaman or magician is someone who can enter this otherworld at will, who has travelled widely in it, knows its language and customs and has made friends, smitten enemies and/or acquired allies and servitors there.
This is important as all magic is of these entities' making. The modern German word for witch, "Hexe" (f.) illustrates this rather neatly if we take a closer look at its etymology. It derives from Old High German "hagazussa" which translates as "fence rider". The hagazussa is riding the "fence between the worlds" i.e. she is at home in the world of everyday life as well as in the magical otherworld of spirits.
In the spirit model magic is seen as being effected by these entities who are usually invisible, at least to the average punter, and it is the shaman's or magician's task to make them put his will into effect. This may be done by prayer, by barter, by cajoling or even - vide medieval demon magic - by the application of magical force, threats and pressure.
The otherworld may have its own geography but it is usually considered to coexist with the world of everyday life. The key to entering it is an altered state of consciousness, controlled trance or ecstasy of which the shaman is an expert.