The Product Backlog is the collection of all product requirements.
The Product Owner is responsible for the Product Backlog, but for the Product Backlog Refinement he involves the Development Team to help him.
Attendees (who)
In the Product Backlog Refinement both, Product Owner and Developers work together in harmony.
When
The Product Backlog Refinement is done before the next Sprint Planning Meeting for a specific set of user stories.
Frequency
Every 1-2 weeks
Duration
1-1.5 hours