Domain Security Policy

A domain security policy would involve a policy set to affect all users who are members of a domain. A Domain Controller security policy affects only users who logon locally to a domain controller.

To expand a little on Boyd's answer (which is correct):

The Domain Security Policy applies to all domain users AND computers other than Domain Controllers.

The Domain Controllers actually exist in an OU called (what else) Domain Controllers. Therefore, the Domain Controller Security Policy applies to the Domain Controller OU (i.e. all Domain Controllers, and anyone who logs on to them locally).

Just as a note: What you see for security policies in Administrative Tools, is actually only a portion of the settings that can be configured. In order to get all settings, you must create a new MMC and add the Group Policy snapin for the appropriate level.