It is the belief evil and sin are different. Both are bad, but evil suggests that there are things out there that are such; that there is an understanding of what it is. Sin suggests that a person's understanding of evil falls short of God's Understanding. Sin suggests that man can't (and won't) do what is right by God.
Evil existed before adam and eve because they became like the gods who already knew of it. And 'sin' means 'to miss the mark' or 'to err' and before adam and eve missed the mark the world was perfect because they didn't have the capacity to do evil(through err) because they didn't have knowledge of good and evil(a conscience) only good, like animals.