XML in full is eXtensible Markup Language.
By definition it is a metalanguage for defining custom markup languages, otherwise known as vocabularies. A metalanguage is a language we use for defining other languages.
XML is very important and popular to the computer science field and this ability to define vocabularies is the key.
So Applications use XML documents to:
Through XML we can define rules for encoding documents that is both human readable and machine-readable.
There are many XML vocabularies but the most common one is the HTML.
XML documents are text-based and comprise markup and content. A markup is the encoded descriptions of a document's logical structure. The content is the document text that is not interpreted as markup.
Markup is evidenced via tags(angle bracket-delimited syntactic constructs) and each tag has a name. Furthermore, some tags have attributes (name-value pairs).