The purpose of this appendix is to introduce you to XML. A knowledge of XML is essential if you want to write RSS documents directly, rather than having them generated by some utility. If you're already acquainted with XML, you don't need to read this appendix. If not, read on.
The general overview of XML given in this appendix should be more than sufficient to enable you to work with the RSS documents that you will be using. For further information about XML, the O'Reilly books Learning XML by Erik T. Ray, and XML in a Nutshell by Elliotte Rusty Harold and W. Scott Means, are invaluable guides, as is the weekly online magazine XML.com.
Note that this appendix makes frequent reference to the formal XML 1.0 specification, which can be used for further investigation of topics that fall outside the scope of RSS. Readers are also directed to the "Annotated XML Specification," written by Tim Bray and published online at http://XML.com, which provides an illuminating explanation of the XML 1.0 specification, and "What is XML?," by Norm Walsh, also published on XML.com.