ActiveRecord is a ORM library, providing a object mapping to relational databases. It's part of the Rails framework though it can be used separately. It is sometimes compared to Apple's CoreData (or the ill-fated EOF), even if ActiveRecord is richer in term of features.

More info: http://wiki.rubyonrails.org/rails/pages/ActiveRecord∞

Very recently, RubyCocoa introduced ActiveRecordSupport, meaning that you can use it as your database backend in the same way as if you were using CoreData.