Unified Content Model
From Joomla! Documentation
The Joomla! CMS is a Content Management System. The Unified Content Model is a concept that involves the organization of content and content related features in the CMS. Joomla 1.x, 1.5 and 3.0 do not used a unified content model. That is, each kind of content (articles, weblinks, contacts, newsfeeds, and others) is considered completely separate from the others. However, a closer look a the database structures and and code for each type of content reveals that there are many commonalities. The UCM concept seeks to take advantage of this.