A function is an assignment or a task that must be performed to complement the other part of a program. There are two kinds of functions: those supplied to you and those you will be writing. The functions that are supplied to you are usually in three categories: those built-in the operating system, those written in C++ and those supplied with your programming environment. The use of these functions is the same regardless of the means you get them; you should know what a function looks like, how to create one, what functions are already available, where they are located, and what a particular function does, how and when to use it.