The law is the Word in which God teaches and tells us what we are to do and not to do, as in the Ten commandments.
…The other word of God is not Law or commandment, nor does it require anything of us; but after the first Word, that of the Law, has done this work and distressful misery and poverty have been produced in the heart, God comes and offers his lovely, living Word, and promises, pledges, and obligates himself to give grace and help, that we may get out of this misery and that all sins not only be forgiven but also blotted out and that love and delight to fulfill the law may be given besides. See, this divine promise of his grace and of the forgiveness of his is properly called Gospel. And I say again and yet again that you should never understand Gospel to mean anything but the divine promise of his grace and of the forgiveness of sin. For this is why hitherto St. Paul’s epistles were not understood and cannot be understood by our adversaries even now; they do not know what Law and Gospel really are. For they consider Christ a Legislator and the Gospel nothing but the teaching of new laws. This is nothing else but locking up the gospel and obscuring everything. For “Gospel” is Greek and means “good news,” because in it is proclaimed the saving doctrine of life, of the divine promise, and grace and the forgiveness of sins are offered. Therefore works do not belong to the gospel; for it is not laws but faith alone, because it is nothing whatever but the promise and offer of divine grace. He, then, who believes the Gospel receives grace and the Holy Spirit. Thereby the heart becomes glad and joyful in God and then keeps the Law gladly and freely, without the fear of punishment and without the expectation of reward; for it is sated and satisfied with that grace of God by which the law has been satisfied.
—Martin Luther, Luther’s Sermons, trans. John Nicholas Lenker and Eugene F. A. Klug, 7 vols (Grand Rapids: Baker, 2000), 1.99