If you want to send a document to people who might not have Word installed on their computers, you can save the document in a more universal file format so that it can be opened in other programs.
Word documents can easily be converted to Web pages. In Web Layout view, you can see how a document will look in a Web browser, and you can make adjustments to the layout from within Word.
The XML format stores information in such a way that it can be extracted and manipulated in a variety of ways in a variety of programs. With Word, you can convert a document to XML with a straightforward save process, or you can use a schema to ensure the validity of the document's structure before saving it as an XML file.