This document discusses user-defined functions in C programming. It provides examples of functions with and without parameters and return values. It also demonstrates call by value and call by reference methods. The key points covered are: 1) User-defined functions allow programmers to define reusable blocks of code to perform tasks. 2) Functions can receive input from parameters and return outputs through return values. 3) Parameters passed by value are copied into the function, so changes made inside the function do not affect the original variables. 4) Parameters passed by reference allow the function to modify the original variables.