Before I bore you with some theory let me quickly show you, how I think you can fill any (!) empty napkin in a minute with a meaningful sketch of a software system, be it completely new or already 30 years old, whether it´s object oriented or all assembler code, and regardless of its size.
Imagine your boss calling you in to talk to you about a new software project. He explains to you his grand vision of a one-size-fits-all "Hello, world!" program. And you can´t hold back your excitement. What a great opportunity for you to show off your skills as a software architect! You go through the requirements with him and make sure you understand every detail. Then you impress your boss with your first architectural sketch. It´s rough, yes, but at least you can show him something right away. You don´t fear an empty sheet of paper (or Visio drawing canvas). Right to the contrary! You love napkins when they are empty!
Here´s what you draw for your boss on the back of a napkin you happen to have brought with you:
That´s it. Plain simple, and enough to convey your understanding of his amazing idea.
And what is it you depicted? It´s the whole application, all that you have to develop and don´t know yet how to implement. All the intricacies and complexities of the application are represented by, well, just one "software cell" - the circle with the dot in the middle:
I call it a software cell, because it looks like a biological cell with a membrane and a core:
source: http://www.schule.at/index.php?url=kategorien&kthid=6191
And like a biological cell a software cell encapsulates complex processes and shields them from the outside world. By drawing a software cell you thus distinguish an inside from an outside, a system from its environment.
That´s not difficult, isn´t it? But it´s an important first step, since it draws a line in the sand separating what you have to implement from what you don´t have to implement. And it´s important because it starts software development on a level of abstraction your boss is still comfortable with. He´s an important stakeholder of the whole effort, so you want him to feel comfortable and confident and understanding as long as possible. How better to do that than by drawing pictures even he understands?