Views 2 is awesome, and the ability to create relationships is one of the reasons why. Essentially, Views is a Query builder that helps you select sets of data from the database without having to understand the Drupal database schema nor write any SQL. If a relationship is not already available for the query you are trying to build you can specify your own by creating a module and implementing hook_views_data(). To do this you create a function called yourmodulename_views_data() that returns an array detailing the relationship you are trying to create. Documentation on how to do this is here: http://views-help.doc.logrus.com/help/views/a... but I didn't really "get it" until I studied hooks provided with Views 2 and some other modules such as Flag.