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. Grace is extending kindness to the unworthy.
"So let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need."