XML (Extensible Markup Language) is a W3C standard for representing structured data as nested tagged text. It is widely used for machine-to-machine communication, configuration, document exchange, and data interchange between systems that need a strict tree-shaped format.
XML itself is only a syntax. Specific vocabularies such as RDF XML, RSS, SVG, or custom application schemas define what the tags actually mean.
Hello world
<?xml version="1.0" encoding="UTF-8"?>
<greeting>
<message>Hello, world!</message>
</greeting>
This is a tiny XML document with one root element (greeting) and one child element (message).
Why it matters here
In the semantic-web context, XML is relevant because some W3C standards use it as their serialization surface. RDF XML is one example: RDF data encoded using XML syntax.