Since we're on the topic of Ghost, let's talk about Spirits and Soul. :D
This is my perspective on them:
Ghost - Residue of the dead. When a person die with strong emotions, they leave an imprint upon the world, that is what that is remain. Their leftover, it is not them but merely what was left behind. The stronger the emotions, the more "advance" the ghost. For example, a person who died with strong emotions could leave a residue that could not do much except going around until it's unfulfilled wish is somehow fulfilled or a person who died with stronger and more intense emotions could leave a residue that might be capable to thoughts and stuff.
Spirit - I don't know much about them but from what I think of them, there are differences. I think that they could think, interact with physical object if they can materialize, live in another dimension really close to ours which would also explain why they cannot touch most physical object and yet can be seen. For example, an angel and a man was walking through a forest, they came upon a large rock and the man walk around it while the angel walk through it. And yet they are both passing through air. (Doubt this would make sense, Can't explain it well,)
Souls - This is you and all living thing that can think. Personally, I classified them into three types. Blank slate, Growth and Final. They represent you in terms of personality, attitude and basically what makes you who you are.
Blank Slate is when life was first made. When you or any other moving living creature are born or made. You are empty, innocent, pure and have a really white soul.
Growth - When colors is added to the soul. When your personality and who you are began to surface. This is when you began to grow and learn, when your soul began to change. Whether you have a dark, dirty soul or a beautiful one is mostly through this.
Final - When you die or reach the point where you can no longer change too much. Your soul is finalize or fully grown, it could be evil, dark, gritty and absolutely disgusting or it could be beautifully colorful and amazing. It is here when you enter the next world that is separated by death.
I could say more but that might take too long. Know that all this is not all of what I think they all, it is only a small portion of it. This is my view, my perspective on the topic of Ghost... with added content :P