Three verses on the topic of mercy are Ephesians 2:4-5, Micah 7:18 and Matthew 9:13. The term "grace" is sometimes used as a synonym for mercy.
In these verses, mercy is the forgiveness of transgressions (sins) given because of the Biblical God's love for his people. This is shown in Ephesians 2:4-5, where it says "...because of his great love for us, God, who is rich in mercy, made us alive with Christ even when we were dead in transgressions -- it is by grace you have been saved." This verse additionally shows an example of grace used as a synonym for mercy.
The concept of mercy shows God not as vengeful but forgiving, as shown in Micah 7:18, where it says, "...You (God) do not stay angry forever, but delight to show mercy." In these verses, mercy is not bought through sacrifice or limited to those who are worthy but given freely by God, as it also is shown in Matthew 9:13: "...I desire mercy, not sacrifice. For I have not come to call the righteous, but sinners."