Documentation. An essential part of any software. Plain human language description of complex software systems that helps non-programmers understand programs, how these programs work, their capabilities and how to achieve desired objectives using the software.
Good documentation is a boon to users, but bad documentation can be downright insulting. It is therefore essential that the users can easily find what they are looking for. To that end it should be organised in a logical manner, easily searchable, extensively linked and indexed.
Help systems vary widely between platforms and in time; man pages on unix, Info files on GNU/Linux, the MacOS help system, various systems for individual pieces of proprietry software, straight HTML, some failed attempts at cross-platform help, custom XML (e.g. DocBook) and several generations of help systems for MS' family of MS-DOS & Windows operating systems. DocBook already plays a huge role as a source format, mainly in book publishing and for software documentation on free/open *nix platforms, while the investment into web browsers during the browser wars and beyond has established HTML as a standard for end-product documentation, whether residing as individual files in a filesystem or inside a custom proprietry format. As a side note that is specification itself was written in DocBook XML and standard software was used to translate the source XML into the several formats that are available, including HTML Help.