According to the Bible, Satan and the demons are angels which have rebelled against God. God did not create the demonic forces as a separate category from angels which were evil from the beginning. Everything God has created was good. But some of those members of God's creation who were given free will, rebelled against him, live in opposition to God, and seek destruction of all that God wants.
In the Islamic understanding, ANGELS and JINN are two different categories of creation. The jinn are mostly feared in Islam, but some are considered to be Muslims. In many respects, jinn in Islam are similar to demons in the Bible, but the concept is not identical.
One of the questions that will come up sooner or later in the dialogue between Christians and Muslims, particularly when the prophethood of Muhammad is discussed, is the question whether Muhammad was inspired or influenced by the dark powers of Satan and the demonic world. See the entry Muhammad and Satan.