What is a Product Backlog?

A list of all the work yet to be done.
A ranked list of briefly-described feature requests. Ideally, these should be broken down into user stories. Learn more.

Why is the Product Backlog Important?

It is constructed and designed so a delivery team can provide estimates for each feature. Learn more.
It helps guide roadmap planning. Having a list of features whose rank can be changed allows us to welcome new ideas and changes in direction. Learn more.
It helps teams focus on delivering the highest value first; teams rank and re-rank for value.

Characteristics of the Product Backlog

One backlog per product, seeded by large to very large features
Backlogs can be worked on by a single team or by multiple teams.
Feature ranking is based on business value, technical value, risk management, and strategic fit.
Highest ranked backlog items are decomposed during release planning into stories that can be completed inside an iteration.