==== XML ====

The well-known **HTML** was the foundational language for web development. HTML is primarily used for **data presentation** but unsuitable for describing data structures. The basic function of HTML is to define how information is displayed on the web (e.g., font size, type, tables, paragraphs). 

**XML** (eXtensible Markup Language) is a **text-based descriptive language** that carries **structural information** about the data. XML is a web-adapted version of **SGML** (Standard Generalized Markup Language ISO 8879). Although XML is sometimes referred to as a programming language, it is actually a **meta-language**, meaning it can be used to describe other languages.

==== Three Key Features of XML ====

1. **Extensible**: You can declare your own elements, making it flexible.

2. **Markup**: It is a markup language because elements are distinguishable by specific tags.

3. **Language**: It has a fixed vocabulary and syntax.

The fundamental difference between **HTML** and **XML** is that you can define your own **tags** in XML. HTML works with a predefined set of tags, whereas XML allows the creation of custom tags.

=== Languages Created in XML for Specific Problem-Solving and Generalization ===

- **XSL**: A formatting language specifically designed for processing XML data.

- **XHTML**: eXtensible HyperText Markup Language. [Learn more about XHTML](http://www.w3schools.com/xhtml/).

- **WAP**: The language for mobile communication. [WAP/WML Reference](http://www.w3schools.com/wap/wml_reference.asp).

- **SMIL**: A standard multimedia description language that enables advanced multimedia and web displays. [Learn more about SMIL](http://www.w3schools.com/smil/smil_intro.asp).

- **XSL Patterns**: A query language that enables efficient searches in XML documents.

- **SVG**: Scalable Vector Graphics for describing 2D vector graphics. [Learn more about SVG](http://www.w3.org/TR/voicexml20/).

- **VoiceXML**: Used for speech recognition, internet voice transmission, and audio compression. [Learn more about VoiceXML](http://www.w3.org/TR/voicexml20/).

- **MathML**: A language for describing mathematical formulas. [Learn more about MathML](http://www.w3.org/TR/REC-MathML/).

This provides an overview of XML's capabilities, applications, and its differences from HTML.