CMS is not really a new word in the internet vocabulary. To describe it in a few lines, a Content Management System is what manages your online content, which can be your blog, forum, articles, gallery, videocast, podcast, magazine or anything. With a bunch of different Content Management Systems already available it’s really difficult to make your own place there.
To name a few, Joomla and Drupal are amongst the most popular CMSes. In fact, WordPress (primarily a blogging platform) also crosses boundaries of being ONLY a blogging platform, which certainly has got many powerful features for being considered as a CMS.