What Is XML? A Complete Guide to Extensible Markup Language

The Purpose of XML

XML was developed by the W3C in the late 1990s to address a fundamental need: a standard way to describe structured data that works across platforms, languages, and organizations. Unlike HTML, which defines how content should look, XML defines what data means through custom tags.

XML Structure and Syntax

An XML document is a tree of elements, each defined by a pair of tags:

<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
  <book category="fiction">
    <title lang="en">The Great Gatsby</title>
    <author>F. Scott Fitzgerald</author>
    <year>1925</year>
    <price>10.99</price>
  </book>
  <book category="programming">
    <title lang="en">Clean Code</title>
    <author>Robert C. Martin</author>
    <year>2008</year>
    <price>35.50</price>
  </book>
</bookstore>

Key structural elements include:

• The prolog: An optional declaration at the top specifying XML version and character encoding.
• Elements: Defined by opening and closing tags (<title>...</title>).
• Attributes: Name-value pairs inside opening tags (category="fiction").
• Nesting: Elements can contain other elements, forming a hierarchical tree.
• Self-closing tags: Empty elements use <br /> syntax.

Rules for Well-Formed XML

XML is strict — a single error makes the entire document invalid:

1. Every opening tag must have a closing tag: <p> must be matched by </p>.
2. Tags must be properly nested: <b><i>text</i></b> is valid; <b><i>text</b></i> is not.
3. There must be exactly one root element: All other elements must be descendants of this single root.
4. Attribute values must be quoted: Both single and double quotes are allowed.
5. XML is case-sensitive: <Title> and <title> are different elements.
6. Special characters must be escaped: Use & for &, < for <, > for >.

XML vs HTML: Key Differences

Feature	XML	HTML
Purpose	Store/transport data	Display content
Tags	Custom, user-defined	Predefined by the spec
Error handling	Strict — any error stops parsing	Lenient — browsers fix errors
Closing tags	Required for all elements	Optional for some elements
Case sensitivity	Case-sensitive	Case-insensitive

Common Use Cases

• Configuration files: Maven (pom.xml), Spring, Android manifests, and MSBuild all rely on XML.
• Web services: SOAP APIs use XML envelopes for structured message exchange.
• Document formats: Office Open XML (.docx, .xlsx) and SVG are XML-based.
• Data interchange: Industries like finance (FpML) and healthcare (HL7) use XML schemas.
• Sitemaps: Search engines read sitemap.xml to discover pages for indexing.