First of all User Defined Functions as specified within the Formulas properties of your app. Just like Named Formulas. In the above example we can see that the structure is fairly similar.

A program (binary, or executable) is a file on disk somewhere, in a recognized format. Common formats include ELF on Linux, ...

The big picture All functions, whether built-in or user-defined, generally can be described as subroutines that return a value or complete a task.