XML is the most popular way to store data and exchange information over the Internet. Since so many languages can read and write XML files, use XML when you want to share data among different applications and platforms. One of XML's greatest features is its ubiquity.
XML also benefits from being easy to learn. Since XML looks like HTML, web developers are familiar with its tag-based syntax. However, XML is not HTML. HTML has a fixed set of elements: <a>, <img>, <h1>, etc. With XML, you have the flexibility to use whatever element names best represent your data.
When choosing how to represent data, developers seem to fall into one of two camps. Some people think of XML as a record format, similar to comma-separated files. But instead of separating entries with newlines and fields with commas, XML provides richer classification options.
Other developers view XML as a document specification format. The online PHP Manual (http://www.php.net/manual) is produced from XML files. The PHP Documentation Team's documents use tags such as <function>, <parameter>, and <example>. This allows them to release the manual in multiple formats, including two versions of HTML, Windows CHM help files, and PDF.