"Shamans have been around since the sentient races first discovered the power of nature. On Draenor, now shattered Outland, the Orcs were shamanic; on Azeroth, Trolls and Tauren were shamanic. Though shamanism on Azeroth flourished and still continues to, shamanism was all but extinct by the time of the great crossing of the Horde onto Azeroth via the Dark Portal. The greatest and yet some of the most vilified Orcs were once shaman; Drek'Thar, Ner'zhul, and Gul'dan were all shamans. However, Kil'jaeden, current lord over the Burning Legion, distorted the shaman's connection with their spirits in the sacred mountain of Oshu'gun, himself taking on the form of the shaman's ancestors to convince the shaman (who held much political sway in the orcish culture) that the Draenei were the enemy. The ensuing massacres upon several Draenei hunting parties offended the spirits, who eventually denied the shaman their powers.
All was set for the transition.
Cut off from their elemental powers, the former shaman turned to the more efficient and masterful powers of the Burning Legion, becoming warlocks. Though many tried to hang onto their roots, it was impossible. Even Drek'Thar was sucked into warlock magic, though he later repented and to this day has never forgiven himself for the acceptance of the perversion that plague himself and the orc race to this day. Ner'zhul, too, reverted to dark magic, though his power was used to escape Draenor and hide from Kil'jaeden.
Shamanism in the orcs died until Thrall, the son of the deceased Durotan, chieftain of the Frostwolf clan, grabbed hold of the reins of Warchief of the New Horde ushering in a new generation of shamanism and outlawing the dark magic of the Burning Legion. Shamanism is now at its peak, as the New Horde itself is led by a shaman of great power in the new orcish kingdom of Durotar."
"Note that all shaman gain their power from the Elemental Spirits, so most shamanic philosophy is the same no matter which race practices it. Previously, it was thought that the rigid disciplines of the Holy Light were incompatible with shamanism, but the recent advent of the Draenei Shamans proves that the Spirits are not so different from the Light. Shamans gain access to their power through contract, not bargain. For this reason to refer to act of using their magic as 'calls' rather than 'spells', since it is a power that is asked rather than taken.
Shamanism is a range of traditional beliefs and practices that involve the ability to diagnose, cure, and sometimes cause human suffering by traversing the axis mundi and forming a special relationship with, or gaining control over, spirits. Shamans have been credited with the ability to control the weather, divination, the interpretation of dreams, astral projection, and traveling to upper and lower worlds. Shamanic traditions have existed throughout the world since prehistoric times.
Shamanism is based on the premise that the visible world is pervaded by invisible forces or spirits that affect the lives of the living. In contrast to animism and animatism, which any and usually all members of a society practice, shamanism requires specialized knowledge or abilities. Shamans are not, however, organized into full-time ritual or spiritual associations, as are priests.
The shamans call upon the Elements in their magic. There are five elements, or Spirits: Earth, Air (also Wind), Fire, Water and the Wilds. At their simplest, the elements may cause an earthquake, summon a storm, conjure fire or finding water. As their most complex, the elements are the very world we live in.
The Spirit of the Wilds the the most complex and least understood of the elements. This element is tied to life- the living things that grow when the other four elements are in harmony. The Spirit is theoretically capable of restoring the dead to life, but no true shaman would ever call upon that- to violate the boundaries of life and death is to deny the very lessons of the Spirits. "
More on Shaman Lore can be found here (external link)