The spirit is that part of us that makes contact with God. It's by our spirit that we relate to God and He relates to us. It's this part of us that makes us different from all other part of God's creation. A flower has a body but no soul or spirit. A dog has a body and soul but no spirit.
Man is body, soul, and spirit.
He's the only part of creation that can have a relationship with God. God's plan for man is that the spirit influences the soul and the soul the body, not the body governing the soul. We're to be a spirit-led people. What happens in many cases is that we get the God designed order reversed. Let me give you an example.