This is something I've been struggling with for a while now, something that I'm hoping someone with a background in mythology, folklore, and/or anthropology might be able to help me with.
I come from a religious background: I was raised as a Christian and as such, my ideas on "spirits" are heavily influenced by the trappings of those teachings. There's a pretty narrow cultural understanding of what a spirit is and what it isn't. Oversimplifying, and not taking into account subtleties in belief across the board when it comes to Christian teachings and disagreements, a "spirit" is usually an incorporeal being, or a facet of being. It's almost universally used to describe demons or angels, but also a facet of the human being, especially when you bring the whole "God is a triune being" thing into it, where the "spirit" is an incorporeal part of us as living things. There isn't - universal agreement as to what that means, as far as I can see.
Many I've heard or spoken with agree, a spirit is not the same as a soul, but the difference between them is less certain. Some believe the spirit to be a sort of repository for things like emotions, desires, things you've learned, biases, etc., stuff that might influence or make up your personality...others equate that to the soul, and not the spirit, and it just muddies the situation up even further. I'm really no clearer on what a spirit is than when I began my research into the matter.
Touching on angels and demons briefly, this becomes even less clear to me...because both tend to have historically corporeal or physical qualities. Depending on who you ask, in Christian tradition, it's possible to mate with and have children with, spirits. You can literally wrestle or grapple them. They have the ability to change their appearance and I guess even their metaphysical qualities, as needed. So that's...interesting...and hard to reconcile with my understanding.
Then I get into other religions, belief systems, myths and legends, etc. Spirits become increasingly less "spiritual," at least when it comes to the area of physicality. Some consider fay and faerie as spirits. Some consider mythological creatures, like dragons, or the griffon, as spirits. Asian traditions have many, many spirits, due to polytheistic and pantheistic ideas, many of which are living breathing "things," and not necessarily wispy, bodyless beings. Obviously the idea of a spirit is going to differ from culture to culture, like anything, but I'm finding very little by way of commonality between them all.
All I can really say, from the meager research that I've done, is that the only thing that is universally shared between "spirits" is that they are (A) more powerful than human beings or normal living things in some regard, often having magical or religious power, and (B) they have a quality that is noticeably inhuman in some way. That's harder to nail down. The importance of their distinction seems to be that they are respresentative of, or are even a manifestation of, something less tangible and more - conceptual? Emotional, maybe? - than a normal living thing is. It's what makes them reverant and worthy of our awe, or terror, etc. But I feel that's missing something fundamental.
It's very hard for me to put this into a question.
I guess I'm hoping someone who has far more experience in this kind of thing can add something meaningful to this, so I can have a better grasp of what a "spirit" is when it comes to studying folklore, myth, legend, culture, etc. Is there some kind of list of qualities that spirits have? I don't know. I hope someone out there can detect what I'm hoping to have answered.