Two team members asked, "What does 'release backlog' mean?" I drew this Venn diagram, and they both understood:
Everything: The universe contains all possible product requirements. Most of them will not be included in our product.
Product backlog: The product backlog contains the subset of all possible product requirements that we agree could be included in our software product. Most of them will be included in the product, but we do not necessarily know when.
Release backlog: We organize our software development work into distinct product releases, and we partition the product backlog a release backlog for each release. The release backlog is the subset of product requirements that we will deliver in a given release.
Sprint backlog: We organize our work into multiple sprints, and we partition the release backlog into a sprint backlog for each sprint. The sprint backlog is the subset of release requirements that we will deliver at the end of a given sprint.