Subroutine:  

A subroutine  is a type of subprogram, a piece of code within a larger program that performs a specific task and is relatively independent of the remaining code.  It is also called a procedure, routine or a method.  A subroutine has no value associated with its name. All outputs are defined in terms of arguments; there may be any number of outputs. 

 In most cases, a subroutine needs some information about the circumstances in which it has been called. A procedure that performs repeated or shared tasks uses different information for each call. This information consists of variables, constants, and expressions that you pass to the procedure when you call it.

  A parameter represents a value that the procedure expects you to supply when you call it. You can create a procedure with no parameters, one parameter, or more than one. The part of the procedure definition that specifies the parameters is called the parameter list. 

 An argument represents the value you supply to a procedure parameter when you call the procedure. The calling code supplies the arguments when it calls the procedure. The part of the procedure call that specifies the arguments is called the argument list. For example here is a subroutine to find the sum of three numbers