At the beginning of the project, the product owner prepares a list of customer requirements prioritized by business value. This list is the Product Backlog, a single list of features prioritized by value delivered to the customer. The Scrum Team contributes to the product backlog by estimating the cost of developing features.
The Product Backlog should include all features visible to the customer, as well as the technical requirements needed to build the product. The highest priority items in the Product Backlog need to be broken down into small enough chunks to be estimable and testable. About ten developer-days of work is a good size for a Product Backlog item that can be ready for implementation in the next iteration. Features that will be implemented further out in time can be less detailed. The Product Backlog can be organized as a Feature List, as User Stories, or as Use Cases. Using Scrum means that you choose the technique that works best for your project.