Shamans are links between human civilisation and nature. This duality is called the Purusha-Prakriti struggle in some Hindu texts, Purusha means man and Prakriti means nature. Human civilisation survives in spite of the existence of the relative brute chaos that is nature. And yet, it can’t destroy nature because that would mean destroying humanity itself. So a constant conflict exists and a balance must be maintained.
A good way to explain the Shaman’s job would be by use of the aboriginal village as an example. The village people need to go into the forest to hunt animals for food, to gather fruits and vegetables, and to cut trees for wood. But if they do it too much they will end up damaging the forest beyond repair and bring ruin upon themselves.
The forest on the other hand, needs its animals to be safe and its trees to stand tall as that is its purpose. Its concerns go beyond the well being of human beings. It is responsible for much more. So it protects itself as much as it can by being a dangerous place. Humans who enter it are frequently mauled by wild animals and fall prey to dangerous diseases. But the forest will not wipe man out as man too is part of the cycle of nature. The Shaman is the link between these two bodies. He is a human who does not live with the others. The Shaman’s hut is often quite some distance away from the village. His job is to be on neither side and yet be on both. He mediates the interactions between man and nature.
The shaman leads human hunting parties into the forest and tells them which parts to hunt in so they do not disturb the forest’s balance of life. He gathers this knowledge by communicating with the forest’s spirits and by his experience as a semi-forest-dweller. The villagers heed his voice in these matters. Thus he helps the forest survive. On the other hand, it is he who heals those who the forest has wounded and helps human society survive the forest’s as well. He does this with help from his knowledge of herbs and drugs that the forest itself provides.
I think this shaman-link is a grave loss in modern societies. Our disrespect for nature stems from this loss. Thousands of years ago, before modern scientists discovered the delicate thing we now call the ecosystem, these forest dwelling tribes understood the value of not killing animals and cutting trees out of arrogance.