Publishing targets are relevant to the Staging module as described in the Staging installation guide. The web database for each runtime server is typically configured as a publishing target. The default publishing target, Internet, specifies the default web database as the target. All content is published to this target regardless of whether that target is selected in the Publishing section of the content item. See http://sdn.sitecore.net/Developer/Understanding%20Standard%20Template/Publishing.html for a little more information on publishing targets.