Field formatters are used to render CCK fields in node views. Drupal provides default formatters. To customize the output of a field and have it apply it to multiple fields of the same type, a custom formatter needs to be created.
The following tutorial is an example of how to add a formatter to the field of type link. To start look at the default formatters available for the link field in a Content Type. In Drupal 7 the available formatters for a field are found by going to Structure > Content types > The content type > Manage display.