What are Resources Today?

See the Resources Wiki.
In short: FCADBS = Files, Containers, Attributes, Deltas, Builders, Synchronization.

Files (==Streams), Folders (==Containers), Groups of them (==Projects) and their attributes (timestamp, encoding, nature, markers, ...)
- Project is a logical group of resources which shares some common attributes and (currently) provides model-specific (nature) interpretation. In Eclipse, projects are always disjoint with each other (except for linked resources == aliases), which enables some concurrency. Projects can reference each other (dependencies).
Caching of attributes (statcache) with explicit refresh, notifications and synchronization, implemented with high performance and low memory footprint.
Resource Delta Notifications (Important: MoveDeleteHook for Team) also for Markers
Support for locking and atomic operations
Hooks for Natures, Model Mappings and Builders. Do these really belong into Resources? Perhaps, in the interest of RCP, these should be split off?
Hooks for Team support and Synchronization
Special resource kinds (Derived, Hidden, Phantom, Team-private)

Additional Layers related to Resources

org.eclipse.core.filesystem - Filesystem (EFS)
org.eclipse.core.contenttype - Content Types
- Encodings and other resource attributes
org.eclipse.ide (the UI side of dealing with resources)
- Working Sets
- Builders and their attributes
org.eclipse.team (team support / synchronization)