What is the difference between a Scrum Master and a Project Manager?

A Project Manager is a leader who manages the project and its team, and is the professional responsible for achieving the project’s objectives.
The Scrum Master is a facilitator and a coach for the Scrum Team, in short: a servant-leader. They ensure the Scrum framework is understood and adopted by everyone and remove impediments to the Development Team’s work. Theirs is more of a coaching and facilitation role.