Internet

In general terms XML is a way of presenting data in a standardized format. 

I would have to suggest that you take a look at the Wikipedia entry for XML.  Some of it is a bit technical, but the "Strengths of XML" and "Weaknesses of XML" portions of the page are pretty good.

XML is often used as a standard for exchanging data

The Extensible Markup Language (XML) is a general-purpose specification for creating custom markup languages. It is classified as an extensible language because it allows its users to define their own elements. Its primary purpose is to help information systems share structured data, particularly via the Internet,[and it is used both to encode documents and to serialize data. In the latter context, it is comparable with other text-based serialization languages such as JSON and YAML.

It started as a simplified subset of the Standard Generalized Markup Language (SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML RSS, MathML, GraphML, Scalable Vector Graphics, MusicXM, and thousands of others. Moreover, XML is sometimes used as the specification language for such application languages.

XML is recommended by the World Wide Web Consortium (W3C). It is a fee-free open standard. The recommendation specifies both the lexical grammar and the requirements for parsing.