Fields on content items are defined at the content-type level, on the <em>Manage fields</em> tab of the content type edit page (which you can reach from the <a href="@types">Content types page</a>). When you define a field for a content type, each content item of that type will have that field added to it. Some fields, such as the Title and Body, are provided for you when you create a content type, or are provided on content types created by your installation profile.