Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format which is both human-readable and machine-readable. 

Example:

<?xml version="1.0" encoding="UTF-8"?>
<Employee>
  <id>001</id>
  <name>Jacob</name>
  <age>31</age>
  <salary>1000000</salary>
</Employee>

Important points about XML

1. XML is both human-readable and machine-readable. 

2. XML was designed to store and transport data.

3. XML is a W3C Recommendation

4. XML is similar to HTML, but they are different

   • XML focus on structure of data, while HTML focus on displaying data

   • HTML has predefined tags, but XML has no predefined tags.

   • XML has a strict syntax compared to HTML.

5. An XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.

   • Unicode is a computing industry standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems.

   •  The latest version of Unicode contains a repertoire of more than 120,000 characters.

   • The standard consists of a set of code charts for visual reference, an encoding method and set of standard character encodings, a set of reference data files, and a number of related items

     • UTF-8 is the default character encoding for XML documents.

Syntax summary

• The characters making up an XML document are divided into markup and content.

  • Generally, strings that constitute markup either begin with the character < and end with a >, or they begin with the character & and end with a ;.

  • Strings of characters that are not markup are content. 

  • E.g. In the above example, id is part of markup while 001 is content.

• A markup construct (tags) that begins with < and ends with >.

  • Tags come in three flavors:

    • start-tags

      • e.g. <section>

    • end-tags

      • e.g. </section>

    • empty-element tags

      • e.g. <line-break />

• A logical document component which either begins with a start-tag and ends with a matching end-tag is called as a element.

  • May consist only of an empty-element tag. 

• A start-tag or an empty-element tag, may contain key/value pairs of data called as attributes.

  • <Employee id ="001">

• XML documents may have a description about itself called as prolog.

  • E.g. <?xml version="1.0" encoding="UTF-8"?>

  • If it exists, it must come first in the document.

  • UTF-8 is the default character encoding for XML documents.

• XML has strict set of rules

  • All XML Elements Must Have a Closing Tag

  • XML Tags are Case Sensitive

  • XML Elements Must be Properly Nested

  • XML Attribute Values Must be Quoted

  • Some characters have a special meaning in XML (e.g. <). They should be replaced with an entity reference.

    • E.g. You can replace the "<" character with an entity reference: &lt;

Please go throgh the attached reference links to learn more about XML.