ghosts: they're trapped. sometimes they don't even realize they're dead. they don't always see us. they're still in that moment when they were alive. the world can go around them, but that doesn't mean they have to see it as it is now. they can be very powerful, and they can be very weak. they can do things to harm you, they can do things to help you. it differs on the person. most often when somebody dies a traumatizing death they don't realize they're dead, but sometimes they are fortunate enough to cross over to a peaceful state of mind known as heaven.
spirits: they've crossed over. they can come in and out... they can watch over us... as some ghosts can.
spirit guides: we all have them. they're known for helping along your intuition. get to be friends with them. ;) some have more than one.
angels: we all have those as well, some of us more than one.
spirit protectors: those are our loved ones that choose to manifest themselves as our protectors in life, as though it is their job to do so. anybody can protect us from the spirit world, though. even those who did us harm in life in this world.
astral projection: your spirit goes places when you sleep, or when you meditate. yes, anybody can do it... hence the whole "OH I'VE BEEN HERE BEFORE!" unless it's...
past lives: some of us have had them, some of us haven't. i'll mention some of mine in due time.