XML is a syntax, but it is mostly a philosophy in writing documents, which consists of describing the meaning in an explicit way as opposed to the presentation. One would write, for instance:
This principle allows both automatic data processing (like the extraction of all the titles to build a table of contents), and the possibility to define several displays for the same content (like a web display and a paper display).
Since the content is well separated from the presentation, XML editors like Jaxe are not WYSIWYG ("What you see is what you get"). People used to word processors like Word have to be careful not to use styles such as bold or italic when it is possible to convey the meaning of the content instead. It is also important to not rely on the preview because there can be several ways of displaying the content. It is better to use elements corresponding to paragraphs or sections rather than line break elements, because the space between paragraphs can depend on the display and be a little less or a little more than a line.