In Visual Basic 2013, sub procedure is a procedure that performs a specific task and to return values, but it does not return a value associated with its name. However, it can return a value through a variable name. A function is similar to a sub procedure in the sense that both are called by the main procedure to fulfill certain tasks. However, there is one difference, a function returns a value whilst a sub procedure does not.
Learn more about them by following the links below:
http://www.vbtutor.net/index.php/visual-basic-2013-lesson-16-sub-procedures/
and
http://www.vbtutor.net/index.php/visual-basic-2013-lesson-17-function-part-1-user-defined-function/