What Is XML? Structure, How It Differs From HTML, Uses, and JSON Comparison

Have you ever seen a run of tags wrapped in angle brackets, like "<name>John Smith</name>," in a configuration file or when exchanging data between apps? That is XML. At first glance it looks a lot like HTML, but XML is a general-purpose markup language designed to "give the data itself meaning and hierarchy so that both people and programs can read it." This article explains, step by step, what XML is, what structure it has, and how it differs from HTML and JSON.

What Is XML

XML (Extensible Markup Language) is a text format for structuring data by wrapping it in tags. It is standardized by the W3C and does not depend on any particular software or programming language, so it has long been used as a common "container" for passing data between different systems.

The biggest feature of XML is that, as its name says, it is "extensible." Whereas HTML can only use predefined tags such as <p> and <a>, XML lets you freely define tag names to fit your purpose, such as <book> or <price>. In other words, XML itself only defines the "rules for writing tags"; which tags you use and what data they represent is up to you.

Another important point is that XML is designed to be readable by both humans and machines. Since the content is plain text, you can open it in a text editor to check it, and a program can walk the hierarchy to pull out the values it needs. To see how XML gets formatted, it is easiest to paste it into the XML Formatter and convert it into a neatly indented, readable form.

Structure of XML: Elements, Attributes, and Text

XML expresses hierarchy by nesting data. The basic parts are three: elements, attributes, and text. Look at the following example.

<book id="b001">
  <title>Intro to XML</title>
  <price currency="USD">19.80</price>
</book>

• Element: a unit enclosed by a start tag (<title>) and an end tag (</title>). Putting elements inside other elements builds the hierarchy. In the example above, the <book> element contains <title> and <price>.
• Attribute: extra information written inside the start tag in the form name="value". In the example, id="b001" and currency="USD" are attributes. They are used to attach small pieces of supplementary information to an element.
• Text: the actual content value sandwiched between tags. "Intro to XML" and "19.80" are the text.

At the very outside, there must be exactly one root element that wraps the whole thing. Many XML documents also place an XML declaration, "<?xml version="1.0" encoding="UTF-8"?>," at the top to indicate the version and character encoding. This idea of "elements, attributes, and text" plus "a single root element" forms the skeleton of XML.

The Well-Formed Rules

XML has minimum syntax rules that a document must follow. A document that satisfies all of them is called well-formed. Most parsers will reject XML that is not well-formed as an error. The main rules are as follows.

• Close every element: a start tag needs a matching end tag. An element with no content is written with a self-closing tag, like "<br/>".
• Nest correctly: tags must not cross. A closing like "<a><b>…</a></b>" is invalid.
• One root element: there must be exactly one top-level element that wraps the whole document.
• Quote attribute values: attribute values must always be enclosed in double or single quotes.
• Case sensitive: <Title> and <title> are treated as different tags.
• Escape special characters: characters like "<" and "&" cannot be written as-is, so they are replaced with entity references such as &lt; and &amp;.

Note that being well-formed is only the minimum condition of "syntactically correct." On top of that, an XML document that has also been checked against a definition such as a DTD or XML Schema to confirm it "matches the prescribed structure" is called valid XML. Being well-formed is the first step. Forgotten closing tags and broken nesting are hard to spot by eye, so it is efficient to format with the XML Formatter while checking for structural problems.

How It Differs From HTML

XML and HTML look almost identical and are easily confused, but their purposes are fundamentally different. Put very simply, HTML describes "how to display data," while XML describes "what the data is."

HTML uses tags with fixed meaning and appearance, such as <h1> and <table>, to tell the browser how things should look. It is somewhat lenient about things like omitted closing tags and tries to render even if the markup is a bit broken. XML, in contrast, lets you name tags yourself but has strict syntax rules, and if it is not well-formed the load itself is rejected. XML deals only with the content and structure of the data; how to present it is left to a separate mechanism (XSLT or processing on the application side).

As a bridge between the two, there is also a standard called XHTML, which rewrites HTML to follow XML's strict syntax. It is easiest to understand as "HTML content written under XML's rules."

What It Is Used For: Config, Data Exchange, RSS, and SVG

Thanks to the versatility of "structured text," XML is still used in many situations today. Let us look at the representative uses.

1. Configuration files

Many applications and frameworks store their settings in XML format. It can express grouped settings as a hierarchy and is easy for humans to open and edit. It is widely used in familiar places, such as Android app layout definitions and build tool settings.

2. Data exchange between systems

When exchanging data between different companies or systems, XML, which does not depend on a specific product, has been prized as a common format. It is still in active use in fields where reliability matters, such as the internal format of standardized office documents, business-to-business transaction data, and various financial and government formats.

3. RSS, SVG, and more

RSS, which distributes update information for news and blogs, is written in XML. And SVG, the vector image format that does not degrade when scaled, is essentially text that defines shapes in XML. In this way, XML is the foundation of many familiar technologies. When XML you receive from a feed or config is hard to read, formatting it with the XML Formatter makes the structure easier to grasp.

Comparison With JSON

In recent years, especially in the area of web APIs, JSON (JavaScript Object Notation) has come to be widely used in place of XML. Both are text formats that represent structured data, but they have different characters.

JSON is more concise because it does not write opening and closing tags, its data size is smaller, and its strength is that many languages, starting with JavaScript, can handle it directly as an object. That is why JSON is now the mainstream for web APIs where the browser and server frequently exchange data.

XML, on the other hand, has strengths for large-scale, strict documents and data exchange: it can carry attributes and comments, validate structure strictly with a schema, and avoid vocabulary clashes with namespaces. There are also many "worlds already built on XML," such as RSS, SVG, and office documents, so it will not disappear. It is not about which is superior; the right answer is to choose according to the use. When you want to compare the look of XML, JSON, and YAML, converting the same data into different formats with the JSON/YAML Converter makes each one's characteristics easy to grasp.

With freely definable tags and strict syntax, XML is a foundational technology that gives data meaning and hierarchy. Start by formatting your XML in the XML Formatter to look at its structure, and compare it with other formats using the JSON/YAML Converter as needed. Touching and comparing them in practice is the shortcut to understanding.

Please note that this article is intended to provide general information. XML specifications, schemas, and related standards can be updated, and the best answer for adopting them in an actual system varies with the requirements. Make your decision after checking the latest official specifications and documentation.