Xerces is an XML parser, which is needed by the tools used in later steps of processing. It converts an XML document into a format that other computer programs can more easily use.
Xalan does the actual conversion of a résumé from XML to other formats. When combined with the XSL stylesheets provided by the XML Résumé Library, it produces directly usable HTML and plain text. It also produces an intermediate format called XSL-FO (XSL Formatting Objects) that can be converted to PDF by an FO processor.