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Determine the business reasons for your documentation. Understand the audience you're writing the documentation for. Determine the appropriate format(s) for the documentation. Decide what form(s) the documentation should take. Choose the appropriate documentation tool.
Although the functional reason for documenting software is to help users understand how to use the application, there are other reasons as well, such as assisting in marketing the software, enhancing the company image, and most notably, reducing technical support costs. In some cases, documentation is necessary to comply with certain regulations or other legal requirements. In no case, however, should software documentation substitute for poor interface design. If an application screen requires reams of documentation to explain it, better to change the screen design to something more intuitive. In most cases, software users have little knowledge of computers outside of the tasks the applications they use enable them to do. There are several ways to determine how to address their needs with your documentation.  Look at the job titles your prospective users hold. A system administrator is likely expert with a number of software applications, while a data entry clerk is more likely to know only the application he or she currently uses to enter data. Look at the users themselves. Although job titles generally indicate what people do, there can be considerable variation in how certain titles are used within a given organization. By interviewing prospective users, you can get a feel for whether your impressions of what their job title indicates are accurate or not. Look at existing documentation. Documentation for previous versions of software, as well as functional specifications, provide some indication as to what the user will need to know to use the program. Keep in mind, however, that end users are not as interested in how the program works as they are in what it can do for them. Identify the tasks needed to do the job, and what tasks need to be done before those tasks can be done. Software documentation can be structured in 1 of 2 formats, the reference manual and the user guide. Sometimes, a combination of formats is the best approach.  A reference manual format is devoted to explaining the individual features of a software application (button, tab, field, and dialog box) and how they work. Many help files are written in this format, particularly context-sensitive help that displays a relevant topic whenever a user clicks the Help button on a particular screen.  A user guide format explains how to use the software to perform a particular task. User guides are often formatted as printed guides or PDFs, although some help files include topics on how to perform particular tasks. (These help topics are usually not context-sensitive, although they may be hyperlinked to from topics that are.) User guides often take the form of tutorials, with a summary of the tasks to be performed in the introduction and instructions given in numbered steps. Software documentation for end users can take 1 or several of many forms: printed manuals, PDF documents, help files, or online help. Each form is designed to show the user how to use each of the program's functions, whether in the form of a walkthrough or a tutorial; in the case of help files and online help, this may include demonstration videos as well as text and still graphics. Help files and online help should be indexed and keyword-searchable to allow users to quickly find the information they're looking for. Although help file authoring tools can generate indexes automatically, it is often better to create the index manually, using terms users are likely to search for. Printed or PDF user manuals can be written with a word-processing program like Word or a sophisticated text editor like FrameMaker, depending on their length and complexity. Help files can be written with a help authoring tool like RoboHelp, Help and Manual, Doc-To-Help, Flare, HelpLogix, or HelpServer.