LibraryGlossaryXML

XML (eXtensible Markup Language) is a general-purpose specification from the World Wide Web Consortium (W3C) for building domain-specific markup languages. Due to its extensibility, users can define their own set of elements.

Throughout its ten years in existence, XML has revolutionized the way that organizations exchange and process data, it has provided the framework for many industry-specific standards, and it has been the vehicle of choice for shuttling data between a web browser and server in Web 2.0.

The purpose of XML is to facilitate the sharing and processing of structured documents and data across different information systems, between organizations, and across the Internet. XML is used to encode documents and to serialize data. It is designed to be both human-readable and machine-processable at the same time.

HTML and XML

When viewed in a text editor, XML looks very similar to HTML. However, XML tags are not predefined as they are in HTML. Just as HTML was designed to display data, XML was designed to transport and store data. Because XML tags describe the data, content and presentation are separate, allowing for documents to be easily repurposed and formatted in different ways. In fact, XML data can be transformed to be displayed in HTML and other formats (more on this below).

XML Example

XML is plain text that uses tags to identify content. For example, a tag that specifies a street might use the start tag <street> and the end tag </street>. The street name would be between these two tags as shown below:

<street>Main Street</street>

XML Elements

Opening and closing tags represent the start and end of an XML element. An XML element includes everything from the start tag through the end tag, including the content between these two tags which may be text, child elements, or both. Attributes of the form attribute="value" can be added to elements to provide additional meta information. The example below has three elements.

XML Attributes

Attributes specify additional information about an element and appear within the opening tag. Attribute values must always be enclosed in either single or double quotes. The attribute in the example below specifies the industry for the company: