Extensible Markup Language - a document encoding markup language.
XML (Extensible Markup Language) is a very widely-used well-supported markup language, upon which many many many document formats have been constructed. Office documents are typically XML-based. As are SVGs, RSS feeds, and hundreds of other types. XML is a form of Data Serialization and is hierarchical (tree-based).
Components
All characters in an XML document are either markup characters or content characters. Markup strings are surrounded by < and > or & and ;. Everything else is content.
Tags
Tags are what’s contained within the less than and greater than symbols. Tags contain a text-based identifier that informs how elements should be parsed. For example <img> identifies an the element represents an image.
Elements
Elements are a a chunk of xml contained between matching opening and closing tags.
Attributes
Attributes are name-value pairs inserted into the tag next to the tag identifier to supply supplemental data, such as the src attribute in an image tag: <img src="whatever.jpg">
XML Declaration
XML documents begin with a declaration that describe how they are formatted (what standard they are using, what character encoding, etc).
Example
<greeting>Hello there, <lie>friend</lie>.</greeting>