"People and Interactions over Processes and Tools."
Regardless of how good a Scrum team is, there is always some room for improvement. The Sprint Retrospective event allows the team to reflect on how they worked together at the end of each Sprint. These can include elements of team motivation and communication.
Who - The development team, Scrum Master and Product Owner (optional). Anybody who actively worked on tasks throughout the Sprint should be part of this event.