When you write programs in Mathematica, there are various ways to document your code. As always, by far the best thing is to write clear code, and to name the objects you define as explicitly as possible.
Sometimes, however, you may want to add some "commentary text" to your code, to make it easier to understand. You can add such text at any point in your code simply by enclosing it in matching and . Notice that in Mathematica, "comments" enclosed in and can be nested in any way.
There is a convention in Mathematica that all functions intended for later use should be given a definite "usage message", which documents their basic usage. This message is defined as the value of f::usage, and is retrieved when you type ?f.
|f::usage="text"||define the usage message for a function|
|?f||get information about a function|
|??f||get more information about a function|
When you define a function f, you can usually display its value using ?f. However, if you give a usage message for f, then ?f just gives the usage message. Only when you type ??f do you get all the details about f, including its actual definition.
If you ask for information using about just one function, Mathematica will print out the complete usage messages for the function. If you ask for information on several functions at the same time, however, Mathematica will give the name of each function, if possible with a link to its usage information.
If you use Mathematica with a text-based interface, then messages and comments are the primary mechanisms for documenting your definitions. However, if you use Mathematica with a notebook interface, then you will be able to give much more extensive documentation in text cells in the notebook.