Welcome to my first tutorial. I hope you can follow along. This tutorial walks you through the basics of creating a CCK field type. This tutorial will target Drupal 5.x.
For those who don't know, CCK is Drupal's Content Construction Kit.
CCK allows you to create custom node types and extend existing node types with custom fields. These custom fields are the building block of complex content types and encapsulate handling for complex data types.