Angels and Demons, God and Satan, good men and bad men--it can all get pretty confusing. What kinds of spiritual beings actually exist? What are their powers? How can they actually affect me? The Bible presents clear answers to these questions, mostly in the stories it tells.
Firstly, God himself is a spiritual being, and there is no spiritual being like him. None have the power he has, none have his absolute goodness, and none have his ability to be everywhere and anywhere at once. He has all knowledge and the supreme power to reward and punish.
Christianity demonstrates how God is 3-in-1, tri-unity, or Trinity. He is described separately as being the Father (of the world, of Israel, and of Jesus himself). He is also the Son, Jesus, who came to earth to save it and is coming again. He is also the Holy Spirit, who comes out of the Father and comforts and convicts the world today. All three exist today and have always existed. Yet, God is fundamentally and absolutely one being.
God saw it fit to create spiritual beings to serve him in heaven. These are called angels. Like the military, the angels have rank and superiority over one another. The only angel mentioned as the "first" or "lead" angel (archangel) is named Michael. However, "angel" fundamentally means "messenger." They are created to worship God and communicate messages to humans.
It seems that at some point before Genesis 3, an angel named Lucifer declared that he was more powerful than God, and many other angels believed him. At that point, those angels were sent to earth to be out of God's presence. These "fallen angels" are commonly called "demons" and Lucifer is now referred to as Satan (meaning "accuser") or the Devil. Satan does not have God-like power, only angelic power. He can be anywhere, but not everwhere, and his ability to torment can be limited by God.
Genesis 2 tells in detail how God created humans. They are a mixture of dust from the earth and the spirit or "breath" of God. Humans are spiritual beings that temporarily reside in a body of dust. We have some of God's spiritual attributes, his goodness, ability to exert authority, and reason. Like some of the angels though, we have all chosen our own way over God's at some point. However, he sent his Son to rescue us from our way. Our belief in Jesus rescues us from the power of Satan and will one day bring us into the presence of God (heaven).