A spirit is, like, your mother, my dad, who've made it. They can come around, but they come around in a loving way because they've already made it to God. Most people make it.
A ghost is someone who hasn't made it - in other words, who died, and they don't know they're dead. So they keep walking around and thinking that you're inhabiting their - let's say, their domain. So they're aggravated with you.
