The Devil or Satan is the angel Lucifer who turned against God and led 1/3 of the angels in a rebellion. He lost that battle but rules over Hell, where according to some theologies he tortures the eternal souls of people who were bad during their lifetimes. He is the essence and representative of evil, and spreads bad feelings around the world.
At the end of days he will fight a battle at Armageddon, and lose again.
Some religions worship him, and are known as "Devil worshippers" or "Satanists".
He is very crafty and likes to confuse humans. The only person to win an argument with the devil was the American statesman Daniel Webster.