XML

XML stands for "Extensible Markup Language". It is a file format with an .xml extension and has a plaintext syntax which makes it suitable for cross-platform environments. Like HTML it is a language of tags to describe content. Unlike HTML which has pre-defined tags, each with a standard for it's use, XML allows the author to create their own tags and XML Schema. Also unlike HTML, XML is usually considered to be a data-structure and is commonly to convey database-like data similar to this:

<Users>
  <User name="Dhruv" age="34"></User>
  <User name="Justin" age="57"></User>
  <User name="Kerry" age="23"></User>
</Users>

Or it can be used for settings like these Apache Maven settings:

<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0 https://maven.apache.org/xsd/settings-1.0.0.xsd">
    <localRepository/>
    <interactiveMode/>
    <offline/>
    <pluginGroups/>
    <servers/>
</settings>

Relation to JSON

XML isn't directly related to JSON, but they are both plaintext file formats to describe data structures. Any comparison is usually an opinion about which is "better" or "easier" to use. In general, JSON is more popular than XML for many uses and almost always has a smaller file size.