Cain was the son of Adam and Eve and the cause of his brother Abel's death. As a farmer, he brought produce as an offering to God. God favored Abel's sacrifice of lambs and Abel's jealousy incited him to kill his brother. God asked Cain where his brother was and Cain responded by saying "Am I my brother's keeper?" (Genesis 4:9) As a punishment God curses him to forever wander, but marked him with a sign so that no on should kill him. Cain has one son, Enoch, and is known as the father of the Kenites. The expression "mark of Cain" is taken to mean the obvious sign of a murderer's guilt.
Sources: Bridger, David. Ed. The New Jewish Encyclopedia, NY: Behrman House, 1976; Navigating the Bible II