Here is a collection of the key terms regularly used in Moodle. Best to learn them now. It will assist your future learning a use of Moodle. One can't learn the concepts without first understanding the words.
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XHTML will eventually replace HTML as the language used to program web pages for display in your web browser. It's actually very similar to HTML, but has stricter rules about the formatting. As the world moves to XHTML web browsers will display pages more consistently, and the web will also be more accessible to people with disabilities.
XML (eXtensible Markup Language) comes virtually unchanged from the ISO 8879 standard SGML for the implementation of markup languages. It is a general purpose specification that aims at representing with simplicity documents with high quality and usability.
Examples of markup languages coming from xml are xhtml, mathml, musicml, graphml and svg. Applications like OpenOffice use the ooxml file format to represent word processing documents, spreadsheets, presentations and charts. This file format is essentially a zip file containing xml documents.
Although it is very similar to xhtml, the html specification is not a xml markup language.