What is a Markup Language?

Markup languages are used by developers of word processing software and world wide web sites.
The developers use numerical or character codes to define presentational properties of text and it is embedded in text to control way text displayed.

SGML (Standard Generalized Markup Language) - A standard for describing markup languages
DTD (Document Type Definition) - Formal specification of markup language using SGML

HTML

HTML (Hyper Text Markup Language) - HTML is an SGML DTD and a collection of platform-independent styles.It indicates various components of a WWW document.
HTML is a superficial description of how browser arranges text, images and push-buttons.
HTML was never designed for such tasks as taking orders from customers, transmitting meaningful medical records, and describing the characteristics of product catalogues.
HTML has no tags for defining a chemical reaction, a shipping method, an order quantity, or a book and thus quality of web searches are limited since there is no way to mark something as a price so it is effectively impossible to use price info in a search.

Limitations of Client side

A mark up language is client side. Client side is where the browser or word processor, etc interpretates the text is such a way based on the tags and code and outputs it the way the text is described. The only way to make a website interactive is to use server side technologies. PHP can be used to link a mySQL database together. Perl can process forms using UNIX sendmail and ASP can link a Microsoft Access database. There are a lot of reasons for using these languages but they are server side and so are primarily for sending data to a server whether it be a order for a product or signing up to a mailing list.