Joomla has tried to create a content manager / CCK for several years now. Though the project has been inactive, there are a few database tables that Joomla still uses, mainly for storing content history. Although the original UCM in its spirit isn’t part of Joomla, similar functionality has been introduced by article fields.
So, with com_content already there what purpose does a new UCM extension serve you'll ask? A UCM gives the flexibility of defining multiple "types" of content. This is very useful so you could create types like Films, Actors, Banners all on the same site. Each type has its own fields, and a list of records from each type can be fetched. You could even relate these items together to build an IMDb clone!