Shamanism is an ancient set of techniques, found in tribal cultures all over the world, that help us do certain things:
Shamanism is rooted in an ancient worldview in which everything is seen as having life, or spirit, or a spiritual dimension. A rock, a tree, the wind--all have spirit and consciousness, though not exactly like human consciousness.
We have a natural connection with the consciousnesses around us. By deepening that connection we grow in power and joy.
In shamanic belief, nobody makes it alone. If you have survived this far, help has flowed to you through your connections with the web of life, whether you've been aware of that or not.
Though you are connected with all beings, there are certain ones who are your "inner circle"--certain animals, trees, stones, places, with which you have a particularly deep spiritual kinship. One of the things we do in shamanic practice is to become aware of those special connections and consciously work with them. This helps us in many ways, and allows us to better help others, and besides that, it is a joy.
As you explore your special connections, it's likely that the first one you will become aware of will be with an animal (or perhaps more than one animal). The animals are very close to us spiritually, and are our natural companions and guides in exploring non-ordinary reality. There are worldwide beliefs in guardian animals or totems or animal allies or spirit friends. They've been given many names. "Power animal" is one name for your guardian animal or animal ally.
Over time, you can also find special plant helpers and spirit teachers and connect with ancestor spirits.
To explore these connections, you need to go into a different state of being; you need to alter your consciousness so that you can experience dimensions of reality that we are usually not directly aware of.
To sum up: A practitioner of shamanism is a person who consciously, voluntarily goes into an altered state in order to explore non-ordinary reality, communicate with spirits (animal, plant, ancestral, and other), and bring back knowledge, power, or healing for themselves and others.
What we call "shamanic journey" is a key way of going into the shamanic state of consciousness. In shamanic journey you "see" inwardly, in a dreamlike way, but you are awake and able to choose you own actions (though not the actions of the beings you encounter). In the altered state you travel through varied and marvelous landscapes found in the Lower World (or "inner earth") and the Upper World. You meet your power animal and other spirits and are able to ask questions about your life, receive teaching and help.
One important thing about shamanism is that it is not a faith-based system, but an experience-based system. Suppose you go into spirit journey, travel to a clearing in the woods by a beautiful waterfall, and meet your power animal, who gives you advice about some struggle you're having, maybe a deep grief in your life. Maybe the waterfall calls you into itself, sings to you, helps your tears to flow in a healing way. And maybe when you come out of the journey you kind of shake yourself, and say firmly, "Of course, that wasn't REAL. That was simply my unconscious sending me messages." Well, that's fine. I would say that other beings in the web of life were talking to you; you believe it was your unconscious. No one demands that you make a "leap of faith." On the contrary -- I and many others would encourage you to take your time in deciding what you believe, and never feel forced into anything. All that matters is that you come to the experience itself with a certain amount of openness, and with respect.
Once you have learned to journey and connected with at least one power animal, you can learn many techniques for healing, divination, and other ways of helping yourself and others, as well as simply exploring various dimensions of non-ordinary reality and of your connection to the Earth. Some techniques are done in journey, others not. For instance, in some healing and divination techniques you may not be journeying but working in ordinary reality--yet in an altered state of consciousness.