OPDS is based on three building blocks that can be used independently from one another, but all share the same technology. Clients and content providers can rely on one or more of these blocks to vastly improve their content distribution.
An easy way to browse, search and acquire content from a large collection. Catalogs are designed to be uniquely curated by their creators, with deep navigation based on links and faceted browsing.
A place to find any content that you've bought, borrowed or subscribed to. Shelves are meant to safely store your content, and help you move between multiple devices.
A message shared between the content provider and an OPDS client. They're meant to do all the hard-work for you – download your content, provide extra metadata, add your shelf and provide a catalog – all with a single click.