In Scrum way of agile development, a release is broken into number of iterations called 'Sprints'. Each sprint is typically of 4-6 weeks (fixed)duration. The duration is kept fixed so as to maintain the rhythm of the team and get quick feedback from customer. During the Sprint, the team works on the list of requirements earlier agreed with Product Owner (called Sprint backlog). A beginning of a Sprint is marked by a Sprint planning meeting and end is marked by a Demo of the product to Product Owner.