What is the mark placed on Cain? "And the Lord put a mark on Cain, lest any who found him should attack him." Genesis 4:15 ESV 


What is the mark placed on Cain?
It seems to have been a mark of warning, not of protection, to those who may think to harm him. The mark was a sign of God's mercy for this murderer. Unfortunately, others warped the meaning of the sign and applied it to themselves later in the same chapter: 

Genesis 4:23-24


  Lamech said to his wives: “Adah and Zillah, hear my voice; you wives
  of Lamech, listen to what I say: I have killed a man for wounding me,
  a young man for striking me.   If Cain’s revenge is sevenfold, then
  Lamech’s is seventy-sevenfold.”


Mankind misconstrued God's mark of mercy to be a mark of license, and had no problem killing others who bothered them. To them it was a mark of "revenge" not mercy. Thus, mankind grew worse and worse, since there was no fear of God in them. So the Flood was necessary, and after the Flood, came God's clear command so we would not make the same mistake:

Genesis 9:6


  “Whoever sheds the blood of man, by man shall his blood be shed, for
  God made man in his own image.


They did not seem to see the mark as a physical mark; it appears that they just applied it to themselves as a matter of principle: "I killed a man, so I am marked by God, too."