Scrum is one of the most popular options of Agile Project Management. It consists of a few clear rules and is easy to explain. In a Scrum-Project you have exactly three roles (not more), namely a Product Owner, the ScrumMaster and the Scrum Team. The iteration cycles (which is typical for agile project management) are called Sprint and usually take between 2 and 4 weeks. At the end of a Sprint there is a finished product increment. Each sprint is concluded with a Sprint Review, where the Product Owner reviews and approves the work results. The Product Owner represents the end customer needs and manages the software development. The role combines product and project management tasks. The ScrumTeam comprises between five and nine members, the ScrumMaster acts as a coach for the team.
Requirements are only defined at a high level (without much detail) at the beginning of a Scrum project, and are not necessarily described in full. However, as a rule a high level release plan is drawn up, which contains the known requirements and a sequence for implementing these requirements in successive sprints. The release plan obviously serves the purpose of balancing time and costs, because even agile projects usually have to consider time constraints and budgets.
