Default List Content Types
In Windows SharePoint Services 3.0, every item has a content type assigned to it, either implicitly or explicitly. You explicitly set the content type of an item by selecting it for that item. However, even items that haven't explicitly been assigned a content type by a user must still adhere to the metadata requirements, or schema, of the list in which they are located. And so the list schema actually functions as the default content type of the list itself.
By design, the base content type hierarchy included in Windows SharePoint Services maps to the various types of lists you can create. When you create a list, Windows SharePoint Services copies a list content type, based on the appropriate base site content type, onto the list. Create an announcement list, and Windows SharePoint Services copies the Announcement site content type locally to the list as the default list content type; create a document library, and Windows SharePoint Services copies the Document content type locally to the list as the default list content type; and so on. So although the user may not be aware of a list having a default content type, it always does.
If you make changes to a list, such as adding or deleting columns, you are also in effect making changes to the list content type included on the list by default.
For more information about the content types included with Windows SharePoint Services 3.0 by default, see Base Content Type Hierarchy.