Nice O'Reilly blog post by Gregory Brown

Understanding ActiveRecord: A Gentle Introduction to the Heart of Rails (Part 1)

Rails is advertised as a model-view-controller web framework. In this two-part article, we'll be focusing on the M in MVC. Specifically, we'll be talking about the object-relational mapping (ORM) software, ActiveRecord, that forms the core of any data-centric Rails application. ORM allows us to think in terms of objects and write in our programming language of choice (in this case, Ruby) while still getting the persistence and optimization benefits of SQL databases.