Types of Spirits
Human Spirits In Visitation
These spirits are the ones of humans who have passed. Most of the time people cross over into the otherworld just as they should. When they appear to the living, it is usually to loved ones and family members, although it can be to favorite places they had in life. These spirits are just in visitation, and not stuck here. They may also visit people in dreams.
Earthbound Spirits
People who have experienced great trauma at the time of their deaths may remain stuck between the physical world and the etheric planes. If their death was sudden, they may not even realize they're dead, or know where to go. These are the ones commonly called ghosts. They may be here due to unfinished business, or out of a fear of going to hell. They could be here because they had their dream home, and don't want to leave it. There are many, many reasons why people can be trapped here.
Nature Spirits & Faeries
Nature spirits are generally found in nature, but some may be household spirits as well. They are generally not seen, and there are many types. They may show themselves as physical beings, spirits, lights, or shadows. Some are human friendly while others are not.
Demons
The Spirits classified as demons these days are thought of as evil, but may not be. They can simply be different from us. They may show themselves as physical beings, spirits, lights, or shadows. During the Witch Trials, the Spirits classified as "Demons" were usually just various types of Faeries or Nature Spirits. True Demonic Spirits are non-human spirits that are too dangerous for any human to deal with safely. They may simply be dangerous to us by their nature, as a rattlesnake is, or they could be intentionally malicious.
Angels
Angels are non-human spirits that aid in various aspects of the running of the cosmos. They are generally friendly toward humans, and there are said to be one or more angels that act as "guardians" for each human being. Angels are not ex-humans, they are non-humans, despite what some sappy and extremely bad television shows say.
Man-Made Spirits
Yes, people can create spirits. Some are non-intentional thoughtforms, and other may be created deliberately. They may have been created as a protector, a spy, to aide the creator's magic, or even to cause havoc to a person or location. They usually have an anchor somewhere in the physical, be it a piece of jewelry or other object. They are normally not seen, but felt.
Shadow Beings
Nobody really knows what shadow people are, but I would venture to say that it is a different way that some ghosts manifest themselves. Some may be human, and some non-human. There is a small shadow animal that resembles a tribble that seems to be fairly common. My old High Priestess would call them schmoos.
Animals
Most animals have no problem transcending from this plane to the next, but every now and then you will see a spirit animal. Most of the time it will be our pets. They bond with us so tightly that even though they've passed, they stay with us. Occassionally there will be spirit animal guardians of a place.
Conjured Spirits
These are generally of the non-human variety, and have been conjured to do a magician's bidding. At times the magician doesn't know how to send it back, or loses control over it. These may or may not be human friendly, and may manifest in a variety of ways.