The Product Backlog

1. The single source of requirements for the product.
2. Prioritized list of requirements arranged as value-creating user stories ordered by business value.
3. The user-stories nearer the top are clear and detailed than those below.
4. User-stories taken into sprint backlog are fine-grained enough to be done in a Sprint.
5. User-stories at the bottom are usually vague in scope. They are called Epics (fairly vague ideas) or Themes (extremely vague ideas).
6. This document exists and evolves all through the product’s lifetime.
7. Editable by anyone but owned and prioritized by Product Owner.

"The product backlog is an ordered list of everything that might be needed in the product and is the single source of requirements for any changes to be made to the product."

At its core, the product backlog is an ordered list of items that is managed by a person with the ability to understand the business’ needs.

The product backlog is used to:

capture requests for modifying a product.  This can include add new features, replacing old features, removing features and fixing issues; and ensure the delivery team is given work which maximizes the business benefit to the owner of the product