The word shamanism is originally from the tungishlanguage in Siberia. Anthropologists began using the word to make it easier for discussions and debates. Mainly cultures have their own name for the shamans, for example the Native American use the word Medicine Men. I will use Shaman for this explanation though. A Shaman is basically an individual for a tribe or community who is charged with the spiritual aspects of a community and helps keep the balance between the spiritual world and physical world.
Basically, Shamans learn to go into an altered states of consciousness and in that altered state of consciousness, go into the spirit world and communicate the multiple spirits there which can help the individual, others, and the community around them.
Now, people in shamanic practices do not necessarily have to be in a tribe or need a lineage. It is a very beautiful practice and allows one to heal themselves and the things around them.
My Father and I are both into shamanic practices. However, him and I both have a lineage too it. but like i said before you do not need a lineage to practice. I highly recommend it if you are interested.
As far as I know for a person to be a "shaman" or whatever title you would prefer to use, one must first have a connection to or calling by the spirits in general or of a particular pantheon. Often in classic shamanism this connection/calling occurs when one undergoes a death and rebirth journey, or else dies. Core shamanism doesn't require one to undergo this journey, and as such is pretty much what Doc described. Classic shamanism doesn't require a lineage either, however it generally requires a lineage, a death-rebirth process, and/or circumstances at one's birth, in one's physical appearance, or in one's life to be considered a shaman. There are things about the shaman that set the shaman apart from the community that is served. Core shamanism, again, doesn't require any of these "set apart" categories, but merely uses the concepts inherent in different shamanic traditions, boiled down, and treated for use by anyone, be they atheistic, Christian, pagan, etc. Classic shamanism tends to ride on one pantheon or to use one pantheon as the main, while core shamanism tends to use all pantheon's equally and not so much care for the tradition.
Personally, I don't consider core shamans shamans, as they don't have the calling/connection that I consider necessary in order to function as a shaman.