This is the rough version of a post I'd like to polish up later. Forgive me for not defining terms, or giving good didactical explanations. There will be a better post later, when I have more time and energy.
I work a lot with systems that talk to other systems. Technically, this puts them in the class of distributed systems, but unlike databases or something similarly complex, it's usually some kind of PC (laptop, webserver), talking to some kind of embedded system (a USB device, a sensor).
Regardless of complexity, these systems need to talk to each other to achieve some goal: obtaining sensor data, changing configuration values, etc.