Saint Michael is one of the seven archangels. The name Michael means 'like God'. He is sometimes identified as the angel who guarded the gate of Eden with a sword against the return of the sinful Adam and Eve.
In Jewish and Christian legend he fights for God and is usually depicted with a sword. In the Book of Revelation it is Saint Michael who throws down Satan, hence the pictures of him killing 'the serpent'. He was generally seen as a protector of good Christians and a destroyer of evil.