Most of the decent writeups I can recall are out of date, now.  However, here is a fairly well-made "What is DNN?" marketing video (created by a marketing company) that's gives such a broad, managerial-level explanation of DNN's features that the old DNN version shown (from 2006) does not really detract from the info presented -- in other words, it's a decent look at concepts and features still present in DNN 4.9.x.  The two-part series is hardly all-inclusive but it's a nice start for anyone unfamiliar with web frameworks.

  Part 1 (6 mins 3 secs):  http://www.youtube.com/watch?v=zeTy8OkAHRE&feature=channel_page

  Part 2 (5 mins 58 secs):  http://www.youtube.com/watch?v=X1t3OD38axo&feature=channel_page

-mamlin