Microsoft Word is a word processing application from Microsoft.
It was originally written by Richard Brodie for IBM PCs running DOS in 1983. Later versions were created for the Apple Macintosh (1984), SCO UNIX, OS/2 and Microsoft Windows (1989). It later became part of the Microsoft Office suite, and Microsoft refers to the Windows version of Word as Microsoft Office Word in this context to indicate its inclusion in the suite, although it is still also sold as a standalone product or bundled with Microsoft Works.
Word document format (.DOC) as of the early 2000s was the de facto standard of document file format. Though usually just referred to as "Word document format", this term refers primarily to the format used by default in Word version 97-2003.
In addition to the default Word binary format, there are actually a number of optional alternate file formats that Microsoft has used over the years. Rich Text Format (RTF) was an early effort to create a format for interchanging formatted text between applications. RTF remains an optional format for Word that retains most formatting and all content of the original document. Later, after HTML appeared, Word supported HTML as an additional full-fidelity roundtrip format similar to RTF, with the additional capability that the file could be viewed in a web browser.
Word 2007 uses the new Microsoft Office Open XML format as its default format, but retains the older Word 97-2003 format as an option.
It also supports (for output only) PDF and XPS format, which is much like an open-source PDF system. However, due to disagreements with Adobe, the "Save As PDF" feature will only be available as a free download from Microsoft's web site, not included in the box.
The document formats of the various versions of Word change in subtle and not so subtle ways; formatting created in newer versions does not always survive when viewed in older versions of the program, nearly always because that capability does not exist in the previous version.