If you think WordPress is only for blogs, your definition needs an upgrade. WordPress can handle multiple authors, specialized content types, and pretty much anything a user would expect from a CMS. WordPress is legendary for being easy to learn and use. It's also got a sweet user interface. If your needs are straightforward, WordPress could easily be the best choice.
Joomla is an extremely fast growing CMS platform, very flexible and very friendly. Joomla! can help you build from simple to extremely complex websites. It can sustain your business in the online world and what is good for you, is that you don't need to have knowledge of programming, not even a bit.
Drupal can do almost anything. It was designed to get new features using tons of little modules. Many of these modules are maintained by professional developers for large clients, but you still get them for free. Of the three most popular CMS, Drupal would be considered the "big guns." Although open source and free, Drupal can be overwhelming to the beginner.
DNN is a program that allows for rapid website development. It is based upon a concept of shared software called open source. Open source software is developed by a community with similar interests and is free shared. DNN is also a Content Management System or CMS. It includes tools for publishing, format management, revision control, indexing, search and retrieval.