This video takees a look at the roles in a project team and provides and overview as to their responsiblities and accountabilities
Important to note that a "Role" in a change initiative (in the case a project) has specific responsibilites for that intitative (as well as generally) and hence provides an underatanding of the key skills, background and knowledge that the person going to fulfill that role needs to have to carry out the role properly.
A Scrum Master has specific needs to deliver a change ina Scrum environment, which is different to a Project Manager in a Project. A person can have both needs in skills etc to fill both these roles in the two different change initiative environments. And that is the consideration - Scrum Master for Scrum, Project Manager for a Project. The roles are not movable/interchangeable - the Person filling them IS