When adding a custom field to a content type, you determine its type (whether it will contain text, numbers, or references to other objects) and how it will be displayed (either as a text field or area, a select box, checkbox, radio button, or autocompleting field). A field may have multiple values (i.e., a "person" may have multiple e-mail addresses) or a single value (i.e., an "employee" has a single employee identification number). As you add and edit fields, CCK automatically adjusts the structure of the database as necessary. CCK also provides a number of other features, including intelligent caching for your custom data, an import and export facility for content type definitions, and integration with other contributed modules.