Many people think that Agile project teams do not need to write documents, especially if they use extreme programming methodology. This is is a common misunderstanding.
Even though, in Agile teams, developers usually have closer interaction with customers, a business requirement document is still a must because it is the guidance on how the product should be built. It sets the direction of all development activities. Teams without a business requirement document or a well-defined one often face disasters during implementation. It can also be used to validate the changes requested by customers to keep everyone's focus on delivering a product that meets the objectives of the business.
At a lower level, user requirements are written as user stories in the product backlog in Agile teams. Because of the close collaboration with customers, developers usually need fewer details written in user stories than on a traditional team. Agile methodologies encourage creating minimal documentation that can accurately guide the team to implement the features, and any documentation beyond that could be a waste of time because a working software provides more value to customers than comprehensive documentation.
As for functional requirements, in Agile teams, you add them to your unit tests and automation tests and run them repeatedly to verify the system's behavior. In this way, you will have the confidence of delivering a working software that meets customers' expectations. Do not write functional requirements in a requirement document because you cannot use that document to perform repeated automatic tests. Also, soon enough, those documents will become obsolete because of the changes to requirements. Another benefit of keeping details of functional requirements in the test code is that whenever you implement a requirement change, you will have to update the test code and this will always keep them up to date.
For TaskAgile applications, the business requirement is to build an open source Trello-like task management application that can be extended and customized by the open source community. In the next section, we will write user stories for this application.