In Midgard everything is content that is stored and managed inside a central content repository. The Midgard repository is an organized collection of MgdSchema objects stored in a specifically structured MySQL database. The repository contains site templates, user preferences, content hierarchies and much more. All these content objects are accessed and managed through the Midgard core API and the language bindings that have been built on top of the API.
As Henri mentions, the Midgard repository clearly resembles to the JCR content repository model. The similarity is strong enough that I find it very interesting to look deeper at where the repository models differ and see which features I like better. Here's a quick overview:
- JCR typing is more flexible. The MgdSchema model makes it very easy to extend the repository with custom object types and the parameter feature allows even further runtime extensibility, but all objects are still clearly associated with a defined type. Midgard does not have unstructured nodes or mixin types that you find in JCR.
- Midgard is less constrained by the hierarchy. In Midgard hierarchies are just a well supported special case of a more generic object linking mechanism. JCR references or event the shareable nodes in JCR 2.0 are not as powerful as the many-to-many relationships that you can easily handle in Midgard.
- JCR is more addressable. As a downside of the above point, Midgard does not support as powerful path-based addressing of content objects as JCR does. The Midgard repository is only partially addressable by paths while in JCR everything has a path. On the other hand all Midgard objects are addressable by their identifiers, whereas only referenceable nodes in JCR can be accessed by identifier.
- Midgard queries are more powerful. The JCR 1.0 query model restricts search criteria to only refer to properties of a single node. Repository implementations like Jackrabbit extend the query model somewhat, but the Query Builder feature in Midgard allows more flexible search criteria to be used.
As a summary I think both JCR and the Midgard repository are good examples of the kind of infrastructure that provides a strong base for building modern content management systems. And Midgard's relationship with the desktop world is an interesting example of how content repository technology isn't really limited to just traditional content management systems.