"Spirits" are real in a sence but it's not what you think. Did you ever notice how "ghosts" live in a place where tradgy befell? They're not souls of people who died, or the aura of someone who died unexpectedly or anything. What they really are are demons who impersinate some one or something that was involved in an actual tradgedy or in a rumored happening.
The fact that they can "enter" peoles body, well yes that's true, but again it's demons and they're possesed, and it's not good. Some people say that it is and somepeople come out unharmed, but you never really know untill you've toyed a bit with this stuff. When someone is possesd it's not two souls in one body it's a demon and someones soul fighting for control, or ones soul giving control over. It's never good.
There are angelic beings, angels, but when people die, if they belive in god, or not, they do not beome angels. In the christian religions, humans who die when they are christians, that is, become higher then angels, they are above them. there should be some reference i could give you but i don't know this stuff by heart.