At the bottom of the Manage fields page for a content type, you'll find this:

In order to add a new field to a content type, you need to provide the following information:

A human-readable name for the field. It will be used in input forms and on displayed content.
All characters are allowed, including spaces, accentuated or non-european characters.
Field name:
A machine-readable name for the field. It is used internally to identify the field and handle database storage. When doing custom theming, it is this identifier that you'll use to refer to that field.
Important: The field name cannot be changed once the field has been created.
Allowed characters: a-z (unaccentuated), 0-9 and the underscore (_).
The length of the field name cannot exceed 32 characters (including the 'field_' prefix that gets added automatically - that is, 26 free characters)
Field type:
The type of data to be stored in that field.
Important: The field type cannot be changed once the field has been created.
The available field types depend on the modules you have enabled on your site. CCK comes with 6 basic field types : Additional modules can be downloaded to handle other field types such as dates, files, images... Look at the CCK project page and the complete list of CCK-related modules.
The form element that will be used to input data for this field on content forms : text input, select list, etc...
Each field type has its own list of available widgets. When selecting a field type, the list of widgets you can select is automatically updated.