What does the Nicene Creed mean when it says that the Father and the Son are of the same substance?

http://en.wikipedia.org/wiki/Nicene_Creed says: > And in one Lord Jesus Christ, the Son of God, begotten of the Father > [the only-begotten; that is, of the essence of the Father, God of > God,] Light of Light, very God of very God, begotten, not made, being > of one substance with the Father; How are the Son and the Father different? In the Nicene Creed it seems that they are separate persons but sharing the same substance. Do Catholics believe that there is a substance out of which God is made? What is the "substance" being discussed here? We cannot say that Jesus is God in the sense of identity, because then Jesus is God and the Father is God so Jesus is the Father, which contradicts the doctrine.