XML is an abbreviation for eXtensible Markup Language which is one of many markup languages devised for different purposes. It was designed primarily to transmit data between disparate computer systems that would normally not be able to interact. It is sometimes described as a bridge language because it can be used as a common language that two computer systems can use to communicate with each other. XML is special because it does not contain predefined tags. XML tags are defined only by the user and this is what makes it an extensible language.
A further advantage of XML as a language is that it is fairly easy for humans to read and understand.
HTML is another markup language that is widely known since it is used for building web pages. While XML and HTML have some similarities in the syntax they use, they are designed to meet different business needs. HTML is designed to display data in a Web browser whereas XML’s objective is to store and transmit data. But they are both forms of SGML, or Standardized Generalized Markup Language, which is itself a standard.
XML is widely used and there are several resources on the web to help people learn basics about the language. One particularly helpful website, W3 Schools, has an excellent tutorial on XML: