Lucifier was an angel of light created by God. He was one of the most powerful, beautiful angels. He became proud, and in his pride said, "Non Serviam" (I shall not serve). This is the original sin, the root of all evil. He fell due to his sin, and thus created himself as Satan and was cast out from heaven. This is the same sin that Adam and Eve committed. As Saint Michael, the archangel, said, when confronting Lucifer, "Who is like God?" It is Saint Michael who cast Lucifer from heaven. Each one of us, when committing sin, thus sets ourselves up as a little God, saying "I shall not serve" and answering St. Michael, "I am like God": saying that we would rather sin and be cast into hell with Satan that serve God in eternal joy. Satan created himself, and if we do not repent of our sin, we follow him into hell.
The devil was created by God as everything is. He was one of the most dignified. But he was proud, refused to worship God and rebelled against Him. God gave all the angels a choice and the demons chose to go against Him.
A Different Perspective
The devil is another element of man-made mythology. For an interesting discussion on the development of the devil as evil personified see the link provided below.
Devil's Answer was created in 1998.
The Devil in Me was created in 1977.
Outwitting the Devil was created in 1938.
My Devil in Your Eyes was created in 2010.
When the Devil Dances was created in 2002.
The Little Devil was created on 1988-10-14.
Dancing with the Devil was created in 1999.
Devil's Meat was created in 1994.
Talkin' Devil was created in 1963.
The Devil's Messenger was created in 1961.
Devil's Force was created in 1997.
The Devil at Your Heels was created in 1981.