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.