A very visual diagram, one that I really like because it is very visual, is the Iceberg Diagram.  It is used to explain that besides the visible items, one should also expect a lot of “hidden” ones. Things that you wouldn’t think of immediately.

This diagram can be used to communicate to business people that -for instance- an e-commerce website entails more than just a logon and product pages as shown in the following example of such an iceberg diagram: 

The above picture contains a rather inspirational form.  The following example is a more exhaustive and precise way of modeling the topics:

It’s not as beautiful a representation as the first image, but it serves a different purpose.  The former is used to communicate the idea, the concept that one should expect a large number of essential topics.  The latter is to really go into detail.  Whichever is the best for the job depends on the context in which it is used…