What is a product backlog and how does it differ from a sprint backlog?

The product backlog is an ordered, prioritized list of everything that might be needed in the product, owned and maintained by the Product Owner. Items are described as user stories with varying levels of detail and are continuously refined. The sprint backlog is the subset of product backlog items selected for the current sprint plus the plan for delivering them, owned by the Development Team. The product backlog is dynamic and evolves throughout the project as requirements emerge, while the sprint backlog is relatively stable during the sprint.