A Resource is something that is tracked as an entity in the system, and which may have a fair amount of metadata and capabilities, but which is not one of the managed "objects" of a collection. These include images (in some cases, where images are first-class collection objects), documents, letters, etc.
Resource Service Current Status
The Resource Service documentation is moved over to the new templates. There are some outstanding modeling questions that need to be addressed by the functionality team and community.