The code you write won’t be a monolithic listing. It will be made up of small segments called procedures and you will work on one procedure at a time. This is called modularized approach of programming.
It permeates the Visual Basic language even the longest applications are written by breaking them into small, well defined tasks. Each task is performed by a separate procedure that is written and tested separately from the others. Procedures are useful for implementing repeated tasks.
The two types of procedures are subroutines and functions—the building blocks of your application.
Subroutines : A subroutine is a block of statements that carries out a well-defined task. The subroutine begins with Sub and name of subroutine and its execution stops when EndSub statement is reached, and control returns to the calling program. It is possible to exit a Subroutine prematurely with the Exit statement.
Functions : A function is similar to a subroutine, but a function returns a result. Subroutines perform a task and don’t report anything to the calling program; functions commonly carry out calculations and report the result. The statements that make up a function are placed in a pair of Function/End Function statements. Because the function must report the result to the calling program, it must have a return type.