In general, Calvin and Luther were mostly in agreement in their reform efforts. Both saw scripture, rather than hierarchy (i.e., bishops in general, and the pope in particular), as the fundamental authority for Christians. For this reason, both believed in the use of the local language in religious services and study and both read meanings out of the Bible somewhat at odds with traditional Roman interpretation (e.g., two scriptural sacraments of baptism and communion, rather than seven).
They differed subtly on the question of Jesus' presence at the Eucharist. Luther, though unwilling to embrace the Catholic doctrine of transubstantiation (that Jesus not only becomes present at the Eucharist but that the bread and wine vanish and only appear to remain present), insisted that Jesus was actually present in the bread and wine. Calvin meanwhile believed communion provided a ritual vehicle for faith to awaken the presence of Jesus in the hearts of worshippers, not in the bread and wine. They held wider differences regarding two issues: free will and the role of the state. Calvin believed free will among humans would compromise the omnipotence of God; Luther believed that the choice of faith could welcome the saving grace of God without compromising God's saving power of election.
Also, Luther believed that a separation between church and state was possible and desirable, whereas Calvin advocated a theocracy, so that no state power could claim authority over the church.