What is XML?

XML: Background

XML (Extensible Markup Language) stores and transmits information, both on and off the Web, using a cross-platform, software- and hardware-independent language standard. Providing an international, vendor-independent syntax for describing information, XML enables businesses to more easily communicate with customers, vendors, and partners—regardless of their computer systems and/or software.

Comprised of a set of tags and declarations, XML is a bit like HTML, but it focuses on providing information about the data itself, versus its format: it tells a computer what the actual data is, not what it should look like. Furthermore, unlike HTML, the set of valid tags can be extended to describe any type of data.
For example, this is how XML would describe the weather in New Orleans on March 18:

<?xml version="1.0"?>
   <weather-report>
    <date>March 18</date>
    <time>08:00</time>
    <area>
        <city>New Orleans</city>
        <state>LA</state>
        <region>South</region>
        <country>USA</country>
    </area>
    <measurements>
        <skies>partly cloudy</skies>
        <temperature>82</temperature>
        <wind>
        <direction>NE</direction>
        <windspeed>6</windspeed>
        </wind>
        <humidity>45</humidity>
        <visibility>unlimited</visibility>
    </measurements>
   </weather-report>

Intersystem communication: Evolving to XML

For many years, disparate computer systems have needed to exchange data. In the early days, custom interfaces were usually developed, requiring intimate knowledge of both of the systems being joined. As more of these interfaces proliferated and duplicated effort, EDI (Electronic Data Interchange) emerged as a standard for creating specific types of interfaces, effectively creating interface “contracts” to which both sides of the interface agree.

However, an EDI format cannot be modified without both sides agreeing to that modification. XML, on the other hand, can be extended to add new elements without invalidating the original interface. Therefore, participating systems can be upgraded independently as needed, and XML standard grammars can be upward compatible. Many companies are now adopting standard XML grammars for electronic data transfer.