Crash Course
Crash Course Computer Science #8: Instructions & Programs
We'll show you how different programs can be used to perform different tasks, and how software can unlock new capabilities that aren't built into the hardware. [10 min, 35 sec]
Khan Academy
Khan Academy: Defining a Procedure
That's why all programming languages let you make your own procedures, also known as subroutines, functions, or methods. A procedure is a reusable collection of statements.