Direct from DrupalDownunder 2012, PreviousNext developer Lee Rowlands explains how to roll your own fields, widgets and formatters (and why you'd want to).
Drupal 7 saw the introduction of fields in core and a strong api to allow you to define your own fields and widgets giving you the flexibility to integrate your own business logic and functionality into the familiar administrative interface exposed by Drupal. Lee Rowlands defines a field, widgets and associated formatters with a real world example.
