Shamans are defined as those who invoke, channel and interact with the Spirit World, occasionally seen as Holy men and women depending upon the religions of the region.
Traditionally, they are very unlikely to have any established organization, with the powers either being inherited or individuals being called to purpose through "signs" or "destiny". Thus there is nothing to say that regardless of how "holy" they were, they may have been entirely separate from the priesthoods (if any) of those regions.
Shamans in RPGs represent either the mysterious side of the poorly understood world of the divine, or they are used in the place of priests and clerics in tribal societies who have tenuously established or particularly simple religious beliefs.
Thematically, they overlap a lot with Druids, but haven't managed to gain the same level of recognition as them.
In BECMI, the Shaman first debuted as the "Humanoid Cleric", a simplified, "thematically appropriate" version of the Cleric that could be applied to various non-demihuman monster races, such as orcs and goblinoids. Aside from a slightly different mechanic of progression, involving increasing the basic EXP needed for the humanoid to advance in its racial class, the shaman used the same general rules as the cleric, but its spell list was altered to fit its motif as the "savage" equivalent. The shaman first debuted in the Master's Edition DMG.
Shamans appeared in Advanced Dungeons & Dragons 2nd edition, alongside the Barbarian, in the splatbook "The Complete Barbarian's Handbook". It was a variant Cleric, much like how the Barbarian of that edition was a variant Fighter, and had the same "pre-Iron Age" cultural baggage. Its abilities boiled down to increased mobility, with similar (but less effective) climbing, running and leaping bonuses as to the barbarian. It can't Turn Undead unless it worships a deity with some control over the dead, and even then it does so at a lesser ability compared to the normal cleric. It also doesn't get bonus spells for high wisdom, can't use scrolls due to being illiterate (at least initially), and is in general a really shitty, underpowered class.
An alternative version of the Shaman was one of the kits introduced in The Complete Book of Humanoids, as one of the Cleric kits. This version was no less shitty, being a physically frailer (D6 hit dice instead of D8) cleric with watered down spellcasting, no ability to Turn Undead, and forsaking all of the Raise Dead spells for access to the far less useful Reincarnation spell instead. The Witch Doctor appeared in the same book as a variant Shaman with some limited access to Wizard spells. It was, somehow, even worse than the shaman was.
3rd Edition D&D
The 3rd Edition Shaman class was released in the Oriental Adventures sourcebook as a replacement for Clerics & Druids, since neither of those were argued to have been particularly "Oriental". It actually did a pretty good job of it too, and made for quite a useful PC class. Though ironically it was not included as available to players in the Rokugan campaign setting, which was the book's "standard" setting.
It had the Druid's Animal Companion as well as the Turn/Rebuke Undead ability (although at reduced level) and the ability to spontaneously convert cure spells from the Cleric; it could also see Spirit creatures at will (both Incorporeal & Ethereal). It had access to up THREE Domains, though chosen from a different list from the standard cleric selection, though the choice is not dictated by any particular deity and some of the domains were exactly the same, so you could get combinations that core clerics would never be able to get.
On the down-side, they don't start with the same weapon and armour proficiencies as either the Druid or Cleric, and despite all of the combat feats, they are still nowhere near as effective as a monk in combat. The best thing to do is to get the better armour proficiencies as early as you can, or you can act in an Arcane equivalent role, since you can always buff yourself up with spells.
D20 Past introduces a class named Shaman that's actually the 3rd Edition Druid in all but name.
4th Edition D&D
Like a Warlord, but less cool and more primitive. Basically, you chant and war dance buffs onto other PCs, while summoning animal spirits.The primary advantage of the Shaman is the very clunky and somewhat broken summoned animal, which is nearly impossible for monsters to destroy. Additionally, the healing mechanic for the shaman gets around the 'use a Healing surge to heal' paradigm, which breaks an important aspect of the game.
A solitary class, they feel no need to seek adventure instead spending their time improving their skills. They usually get pulled into trouble anyway due to their usefulness. They are most commonly known for their wide use of the elements and not to be mistaken for Druids. If they feel the need they will use totems, usually hung around the neck or wrist, to help channel their power.
One of the most popular things Shamans are known for is their ability to summon various creatures. What a shaman can summon depends on his dedication to the art and luck. Most creatures summoned by the average shaman will look half put together and often gruesome, adding on a small fear factor to the otherwise smaller and less intimidating shaman. Occasionally you'll run into a more accomplished one who can summon all sorts of things; if it doesn't look like it'll kill you, run.
In group mechanics, shamans are most likely used for their buffing and healing abilities. The buffs shamans have to offer span everything the elements touch, from boosts to resistances. While their healing abilities aren't top of the line, it's hard to argue against someone who can also summon a creature that will eat you.
Shamans are a hybrid class, combining some of the features of the Witch and Oracle classes. Although they are Divine spellcasters like the Oracle, they prepare their spells in advance by communing with their spirit animal like the Witch. They can take the basic Witch hexes (though no major or grand hexes) as well as some of their own. It also gets a spirit class feature and a wandering spirit class feature. The spirit functions similarly to any other caster specialization such as a Sorcerer bloodline, an Oracle mystery, or a Witch patron: it grants some extra spells not usually on the class's list, some extra hexes to choose from, and a few other powers. The shaman is rather versatile in that it can also take a wandering spirit and wandering hexes. With the wandering spirit, it can temporarily take on most of the features of that spirit including its additional spells and its spirit powers (although you cannot take hexes granted by a wandering spirit, its true spirit ability, or its manifestation) and wandering hexes: you can temporarily add certain hexes to your hex list including those from a wandering spirit.
Extra notes go out to the use of the Wandering Hex ability and the Lore spirit. At the start of the day, you can trade out your wandering hex. One of the hexes for the lore spirit allows you to add wizard spells to your list to prepare. You can add any wizard spells to your list. The wandering hex lets you trade them out day by day. Essentially, you've got the entire wizard spell list, alongside your own. The cost is that you need Intelligence, Wisdom, AND Charisma for this ability, but it's a fair trade if you can get your stats high enough.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|