Read psalms 51:1  King David sinned against God.  So God sent Nathan, the prophet to tell David that he was guilty.  David had two choices:  He was the king and could have done anything that he wanted to do.  He could have gotten mad and had Nathan thrown out of his palace.  But David did the right thing,  He was sorry for his sin and repented before God.  This entire psalm is about David's sorrow and repentance.  There are two words that are similar, but very different, and both describe God's goodness  and forgiveness.  They are grace and mercy.  David asks for God to have mercy on him here in our verse.  Mercy and grace are like the two sides a coin: they are different sides, but still the same coin.  Grace means that God gives us something good that we don't deserve.  God gives us forgiveness and a home in heaven when we die.  Mercy means that God gives us forgiveness and does not give us punishment in hell that we really do deserve.  God's grace gives us His goodness that we don't deserve and God's mercy gives us forgiveness that we don't deserve.  Either way, God is good.  When you get into trouble and deserve a spanking, do you need mercy or grace?  When your mother takes you to get a cookies-and-cream blast at Sonic are you getting mercy or grace?  I am glad for both God's mercy and grace!