In a Nutshell:

Well-formed XML conforms to these four rules, determined by the World Wide Web Consortium ( W3C):

1. An XML document consists of a root element, optionally preceded by a prolog and followed by miscellaneous content (which simply allows white space to exist there);
2. XML elements have either
   • An opening tag, data, and a closing tag; or
   • A single self-closing tag.
3. XML tags begin and end with angle-brackets < > and have a name.
   • Opening tags may have attributes;
   • Closing tags have a forward slash immediately after the first angle-bracket </;
   • Self-closing tags have a forward slash immediately before the second angle-bracket />
4. Elements can contain other elements, but they cannot overlap. The first element to open is the last to close.