This is probably the best shortest explanation of the all important distinction between God's law and God's gospel that I've read. It's from Mike Horton's new book The Christian Faith:

In the Reformed tradition, the law-gospel distinction was interpreted within the historical context of distinct covenants in history. The covenant of creation (also called the covenant of works or law) was based on the personal performance of all righteousness by the covenant servant. The covenant of grace is based on the fulfillment of all righteousness by our representative head and is dispensed to the covenant people through faith in him. There is still law in the covenant of grace. However, it is no longer able to condemn believers but directs them in lives of gratitude for God's mercy in Christ.

As I've said here before, the commands in the Bible are like a set of railroad tracks. The tracks provide no power for the train but the train must stay on the tracks in order to function. The law, in other words, never gives any power to do what it commands. It shows us what a sanctified life looks like but it has no sanctifying power. Only the gospel has power, as it were, to move the train. This is why the Bible never tells us what to do before first soaking our hearts and minds in what God in Christ has already done.