Mercy and Grace are two words and two subjects commonly used today but their meanings are often misunderstand or under-appreciate. Mercy and Grace are really sisters. They're not identical twins but they are very close sisters, nonetheless. They are almost constantly found together and if one is encountered by herself, the other is not far alway.
Mercy is God not punishing us as our sins deserve, and grace is God blessing us despite the fact that we do not deserve. Mercy is deliverance from judgement.