Scrum is one of the most used Agile methodologies, and we often refer to roles in Agile when we speak of Scrum’s roles. According to Scrum, there are three primary roles: the Product Owner, the Development Team and the Scrum Master.
The product owner (PO) represents the voice of the customer, defining the features and scope of the product.
It is the main responsibility of the PO to ensure that the team delivers business value at the end of each iteration or sprint, and he or she does this by writing customer-centric user stories, ranking and prioritizing these stories, and adding them to the product backlog.
The team is made up of some number of developers and other specialists who contribute to executing the work and delivering value.
You don’t want too many people on any team. According to the 2-Pizza rule, if you can’t feed your team with two large pizzas, your team is probably too big. Scrum teams are self-organized in that apart from any “management” they decide when to meet, who does what and how to implement the PO’s stories.
The Scrum Master (SM) is a servant-leader responsible for removing all roadblocks that prevent the team from reaching their sprint goals.
Scrum masters are keepers of the rules of Scrum (and other Agile methods), and they enforce them in order to keep the team insulated from external interruptions and operating proper Scrum processes. The SM may facilitate team meetings such as the daily standup or the sprint retrospective. Many scrum masters encorporate coaching and mentoring in their role.