A product backlog is a list of items to be done. Items are ranked with feature descriptions. In an ideal scenario, items should be broken down into user stories.
Each product should have one product backlog which can have a set of large to very large features.
Multiple teams can work on a single product backlog.
Ranking of features is done based on business value, technical value, risk management or strategic fitness.
Highest ranking items are decomposed into smaller stories during release planning so that they can be completed in future iterations.