In short, it gives you Collections with Descriptions, custom Facets, Showcases & Display themes.
Information created in Atrium is written to a database via ActiveRecord not Fedora
Code is in a Rails 3.1 Engine
Blacklight is a dependency, but devs have made an effort to make Atrium freestanding as much as possible
Collection: basically a saved search with description
Showcase: container for descriptions & layout – injected at any point, can be rendered anywhere in template
Exhibit: container for nested facets
Engine will install and you'll have some documentation. Next step is to work on extracting out the parts that would make your own application. This would be in the form of generators.
Curators who are building collections. Not targeted to archivists. Could also be students that are interested in building their own collections as well.