As opposed to spending weeks or months detailing requirements before initiating development, agile development projects quickly prioritize and estimate features, and then refine details when necessary. Features for an iteration are described in more detail by the customers, testers, and developers working together.
Delivering working, tested features is an agile development team's primary measure of progress. Working features serve as the basis for enabling and improving team collaboration, customer feedback, and overall project visibility. They provide evidence that both the system and the project are on track.
It is a myth that agile methods forbid up-front planning. It is true that agile methods insist that up-front planning be held accountable for the resources it consumes. Agile planning is also based as much as possible on solid, historical data - not speculation.
Many agile development teams use the practice of relative estimation for features to accelerate planning and remove unnecessary complexity. Instead of estimating features across a spectrum of unit lengths, they select a few (three to five) relative estimation categories, or buckets, and estimate all features in terms of these categories.
Agile development methods focus rigorously on delivering business value early and continuously, as measured by running, tested software. This requires that the team focuses on product features as the main unit of planning, tracking, and delivery.