The product backlog is an essential artifact in Scrum. It is an ordered list of ideas for the product, kept in the order in which the team expects to do them. It is the single source from which all requirements flow. This means that all the work the development team does comes from the product backlog. Every feature idea, enhancement, bug fix, documentation requirement -- every bit of work the team does -- is derived from a product backlog item. Each item on the product backlog includes a description and an estimate of the team capacity required to produce it.