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XML, or Extensible Markup Language, is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. It was designed to store and transport data, and it is commonly used for representing structured information in a text format. Here are some key features and concepts related to XML:

1. Markup Language: Like HTML, XML uses tags to define elements within a document. However, unlike HTML, XML is not predefined and allows users to define their own tags.
   

2. Elements: XML documents consist of elements, which are defined by tags. An element typically has an opening tag, content, and a closing tag. For example:
   

   xml
   

• <person> <name>John Doe</name> <age>30</age> </person>
  

• Attributes: Elements can have attributes that provide additional information about the element. Attributes are specified within the opening tag. For example:
  

  xml
  

1. <book ISBN="123456789"> <title>Sample Book</title> </book>
   

2. Hierarchy: XML documents have a hierarchical structure, with elements nested within other elements. The hierarchy defines the relationships between different pieces of data.
   

3. Well-Formed XML: An XML document must adhere to specific syntax rules to be considered well-formed. This includes having a single root element, proper nesting, and correct usage of tags.
   

4. DTD and XML Schema: Document Type Definition (DTD) and XML Schema are used to define the structure and the legal elements and attributes of an XML document. They provide a way to validate that an XML document conforms to a specific structure.
   

5. Namespaces: XML namespaces are used to avoid naming conflicts by allowing elements and attributes to be identified with a namespace prefix.
   

Here's a simple example of an XML document:

xml

<library>
    <book ISBN="123456789">
        <title>Sample Book</title>
        <author>John Doe</author>
    </book>
    <book ISBN="987654321">
        <title>Another Book</title>
        <author>Jane Smith</author>
    </book>
</library>

XML is widely used in various applications, such as configuration files, data interchange between systems, and representing data in web services (SOAP and RESTful APIs).