CoreData is an Apple framework that provides serialization of your objects into XML, binary blobs or SQLite databases.

More information on the following external resources:
http://developer.apple.com/macosx/coredata.html∞
http://en.wikipedia.org/wiki/Core_Data∞

RubyCocoa provides out-of-the-box support for CoreData, but also for ActiveRecord, a similar library that is part of the Rails framework.