After 2 years in a BS IT program with an emphasis in business systems analysis, we finally get into the nitty gritty of UML (Unified Modeling Language) diagrams. The light bulb finally went in my interaction-related use case and sequence (or swim) diagrams.
We start with class diagrams as blueprints that describe objects as well as their attributes and operations. It's all about behavior and data.
We can consider state and activity diagrams to see how these object exist through time.
To better understand how the parts come together (as well as roles and security concerns) we can add use case diagrams (and their text-based use case descriptions) as well as sequence diagrams. How do the objects and users interact with each other?
Each diagram would be appropriate to different audiences and contexts, but together you get a nifty toolkit that can be used in a variety of scenarios.