An app is a package of decisions that a person interacts with. Users 

View / Interface Business Logic Data Infrastructure

A coffee shop as an app. A barista's job can be modeled. A chef, can be modeled - just think about the movie, Ratatouille. A law-firm as an app. It makes you think about the systems we have built around systems (think legal system).

The important thing to realize is that world can be modeled. And although the we model the world, it doesn't necessarily mean the model is correct. Rather, the model is a tool, to improve understanding. I find this especially important for teams.