The Product Owner acts as the focal point for all information about the product and its requirements. He or she creates an objective for each iteration and defines a priority for each story. The Product Owner also defines acceptance criteria for stories, which are used by developers to measure their progress toward delivering those stories.
The Scrum Master acts as an interface between developers and management. He or she helps guide teams through all aspects of implementing Scrum, including coaching and mentoring new members of the team. In addition to supporting developers during sprints and meetings, he or she serves as a liaison between developers, testers and stakeholders at all levels of an organization.