Read this:
http://www.w3schools.com/xml/xml_usedfor.asp
XML was designed to describe data. In the real world, computer systems and databases contain data in incompatible formats. One of the most time-consuming challenges for developers has been to exchange data between such systems over the Internet.
Converting the data to XML can greatly reduce this complexity and create data that can be read by many different types of applications.
Since XML data is stored in plain text format, XML provides a software- and hardware-independent way of sharing data.
This makes it much easier to create data that different applications can work with. It also makes it easier to expand or upgrade a system to new operating systems, servers, applications, and new browsers.
Example:
<books>
<book>
<index>1</index>
<title>Title</title>
<author>Author</author>
<category>Category</category>
</book>
<book>
<index>2</index>
<title>Title2</title>
<author>Author2</author>
<category>Category2</category>
</book>
<book>
<index>3</index>
<title>Title3</title>
<author>Author3</author>
<category>Category3</category>
</book>
</books>