CCK is an acronym for Content Construction Kit, a contributed Drupal module that has been included in the OSU Drupal 6 installation profile.
Through the use of CCK, a permitted user can change workflow settings and customize fields on an existing content type, or create entirely new content types as needed. This can be handy for cataloging content while also ensuring that the display of the cataloged content is consistent with others like it.
The CCK module is one of the most popular, well-maintained, and extended contributed modules at drupal.org. In fact, it has proven to be so useful that the module has been absorbed into the Drupal 7 core installation, under the name Fields.