“The interface is the application.” You hear that a lot when someone’s trying to remind you to consider the end-user’s perspective. It’s a good reminder, too — to the user, the inner workings of your application are probably about as interesting as the inner workings of a warehouse. N-tier? Components? Who cares? At the end of the day, if people don’t use your software, is it really any good?
It’s important, though, to remember how this statement is used: it’s a context-shifter, a facetious phrase intended to jar you into thinking — not a statement of fact. Accepting it as factual, and making decisions based on it, is like saying your skin is the entire organism, or that your car seat and dashboard are the whole car. I rarely think about the transmission (unless it’s not working), but that doesn’t mean that the radio moves the car.