But it defines many activities and roles that are not how I have seen the most effective teams organized. And as a framework, it is a very rigid one. From the Scrum Guide:

"The Scrum framework, as outlined herein, is immutable. While implementing only parts of Scrum is possible, the result is not Scrum. Scrum exists only in its entirety and functions well as a container for other techniques, methodologies, and practices."