So far I've been spending my time here setting the stage for introductory spirit work. But what about the spirits themselves? Who and what are these incorporeal beings that we're trying to communicate with in the first place? The answers to those questions, in complete, are much longer and more complex than I can cover in one article. So consider this a very brief and incomplete overview of some of the sorts of beings in the spirit world.
Ancestor Spirits: These are the spirits of humans who have passed away. Some people feel the ancestors still exist in this world with us, unseen but around us all the time. Others believe they inhabit their own afterlife-world, but can be called on for advice or help in times of need. Depending on who you talk to, any family member, friend, or other important person in your life can be considered an ancestor—in fact, since we are all of the same species we all share ancestors. Others are more strict, keeping it specifically to one's own immediate family and direct predecessors. Ancestor spirits are generally seen as protective in nature, though they can be angered if not propitiated properly (and, not surprisingly, what is considered “proper” is not universal).
Nature Spirits: This is a fairly broad category, since it includes not just the spirits of animals, plants, and fungi, among other natural things, but also spirits which are more abstract representations of natural forces. For example, one of the main parts of my practice involves working with animal totems. These are not just individual animal spirits, such as a gray wolf or a least tern, but archetypal beings that embody all of the qualities of the species Gray Wolf and Least Tern. Some people also consider faeries and devas (often taken out of their original cultural contexts) to be nature spirits, though the fey are also often thought to have originally been ancestor spirits, and the New Age conception of devas has deviated quite a bit from its Buddhist and Hindu roots. (This, incidentally, is part of why mythology and other cultural materials need to be part of your spiritual research!)