XML (Extensible Markup Language) is a markup language that provides a format for describing data and is widely used for data exchange between varying computer systems. XML documents are made up of elements, which are further broken down into attributes, content, and values. XML documents can be used to represent rich and complex data, providing a way to store, transfer, and share data among different applications. Data stored in XML can also be easily transferred between different computer systems, making it an excellent choice for data sharing.

XML was created by the World Wide Web Consortium (W3C) in 1998, as a way to make documents easier to read over the internet. It builds upon another markup language, SGML (Standard Generalized Markup Language) and borrows heavily from the HyperText Markup Language (HTML). XML differs from HTML, in that it is considered a "meta-language", allowing the user to create their own custom tags. This makes XML very powerful and versatile, allowing users to represent a wide variety of data and meaning.

XML documents are organized into hierarchical structures and further divided into elements that contain attributes, content, and values. Element tags provide the structure of an XML document, and can include attributes that further provide specific information about a particular element. Contents provide narrative, text, or other information about a particular element, and values are the specific data applied to an element. XML documents are able to support multiple formats, languages, and platforms, allowing developers, users, and machines to all understand the same data, regardless of platform or language.

XML is used in a wide variety of applications, such as web development, data storage, and mobile applications. XML can be used to store user data in a structured method, and can then be used to share data across applications, platforms, and networks. XML can also be used to define the application logic and user interfaces, allowing web developers to maintain a consistent web environment. Web services can also use XML to transfer data and messages across hosts, making it an excellent way to exchange distributed data.

Due to its extensibility, XML provides a powerful, lightweight, and flexible platform for data exchange. Its ability to provide structure and meaning to data makes it an invaluable component in many web applications, making it an important technology for web developers to learn.