In this video series from codeigniter.tv, Joost van Veen introduces MY_Model, the foundation of good Codeigniter extensions.
Joost describes MY_Model like this:
Bascially, a MY_Model is a base model from which you can extend all other models. In your MY_Model you would typically define all basic CRUD (Read, Update, Delete) functionality.
Two videos are currently up:
and more are on the way.