Elm is a purely functional language, meaning all you write are the insides of functions that take a value and return a value. There's no imperative way to "do anything" like make an HTTP request, all you can do is return a "command" value from a function and the core libraries will do it.

This means that the core libraries define the signatures of the functions that an Elm application must implement. (All Elm apps "work the same way" in that sense.) It also means that you never have to mock or stub anything in a test. There are also great implications for performance and maintainability.

Elm compiles to JavaScript and is specific to browser applications, but there's no theoretical reason the same idea couldn't be applied to mobile apps or CLI applications or webservers or anything else. (It's just a lot of work.)