Cain and Abel
In the 5.13 The Song Remains the Same, the Archangel Michael takes John Winchester as a temporary vessel. He explains to Dean that he has many potential vessels that are suitable to hold him because of their bloodline, which stretches all the way back to Cain and Abel.
Castiel, speaking of his early life, mentions his memory of Cain and Abel in 6.20 The Man Who Would Be King.
Cain and Abel in Lore
Cain and Abel are the sons of Adam and Eve. Cain is a crop farmer and his younger brother Abel is a shepherd. Both make offerings to God - Cain of grain and Abel of a lamb. God accepts Abel's offering, but rejects Cain's. This angers Cain and he kills his brother. When God asks Cain where Abel is, Cain replies "Am I my brother's keeper?" When God discovers the murder he puts a mark on Cain so everyone will know he is a murderer, but so that no one will kill him. Cain is banished.
Why God rejected Cain's offering, and whether Cain killed Abel out of jealousy, are subjects of much theological discussion. In some versions of the story, it is actually a dispute over who they are to marry that causes the fatal argument.
The mention of Cain and Abel in Supernatural is likely used to emphasize the fateful nature of the conflicts between Sam and Dean, and Lucifer and Michael.