Any text enclosed between less-than (<) and greater-than signs (>) is a tag called markup. HTML is a greatly simplified version of SGML and it was developed by the World Wide Web Consortium (W3C). HTML mixes ordinary text with special strings of characters used to instruct browsers how to display HTML documents. Web browsers are created specifically for the purpose of reading HTML instructions and displaying the resulting Web page.
Popular Markup Languages
XML stands for Extensible Markup Language, is encoding both text and data designed to transport and store data. XML is designed to self describing, that is tags are not predefined and you should define your own tags.
XHTML is based on XML, and permits site designers to easily add new tags and extensions to their languages. It acts as a bridge between XML and HTML
Dynamic Hyper Text Markup Language can be described as a combination of several technologies like HTML client-side java script and cascading Style Sheets.