User-defined functions are those functions which are defined by the user at the time of writing program. Functions are made for code reusability and for saving time and space.
The arguments that are passed in a function call are called actual arguments. These arguments are defined in the calling function.
The formal arguments are the parameters/arguments in a function declaration. The scope of formal arguments is local to the function definition in which they are used. Formal arguments belong to the called function. Formal arguments are a copy of the actual arguments. A change in formal arguments would not be reflected in the actual arguments.