Hello Dmitry.

FWIW I've had great success over the years with various UML (Unified Modeling Language) diagrams for what you are doing.

One of my all-time favorites is the data flow diagram as it makes it very easy to convey details on the processes/programs (circles) and data stores/database (horizontal lines above and below the text), as well as the logical process and data flow throughout an application (the arrows and connectors). Usually I'm able to depict even large, complex systems on a single page and it allows for presentations to both technical and business types that really help get them familiar with the system quickly.

I find DFDs are great for providing the business folks with a representation of the application they are working with or asking to have built, and usually drives very informative and productive discussions during business requirements gathering meetings and the like. I've also used them to bring developers and other technical staff up-to-speed very quickly on applications they needed to be productive on but had no prior knowledge of.

I'm attaching a very simple example here (didn't want to scare you); feel free to contact me if you'd like to see some more complex ones that are great examples of what I'm talking about. I use Visio for most of my modeling as it has support for most the UML diagram types.

Hope this helps!