XML stands for EXtensible Markup Language.  It is a self descriptive markup language.  This means that the tags used to describe the content of the XML file are not predefined, but instead the author defines his own tags and document structure.  XML was designed to transport and store data, while HTML was designed to display data.  Unlike HTML which has standard tags that browsers can be programmed to display (i.e., <p>, <table>, <h1>, <em>),  an XML file requires software to be written in order to send, receive, display or manipulate the data in the XML file.

XML was created to simplify data sharing.  It provides a method for storing, sending, and receiving data between systems that may contain data in incompatible formats.  Since XML is stored in a plain text format, an XML document is both software and hardware independent. This makes it much easier to create data that different applications can share.

While HTML can display data on a webpage directly, XML allows for easy separation of data from presentation information so that if the data needs to be changed the HTML page is left unaffected.  This can be achieved by using Javascript within the HTML file that references the data in a separate XML file.  Whenever the data needs to be changed, only the XML file needs to be updated.