Explain the structure of XML document.

XML (eXtensible Markup Language) is a markup language used to store and transport data, used in web applications, databases, and other applications. XML stores information in plain-text format, therefore it is easy to read and understand. An XML document is composed of an XML declaration, the root element, elements, and attributes. The XML declaration is the first line of an XML document and contains information about the document such as the encoding used and the version of XML being used. It is optional, but when present it must be the first line of the document. The root element is the top element of an XML document, and all elements in the document should be contained within the root element. Elements are the fundamental building blocks of an XML document and are used to represent components of the data being stored. Elements consist of three parts: the start tag, content, and the end tag. The start and end tags indicate the beginning and end of the element, and the content is the data contained within the element. Attributes are name-value pairs contained within start tags and are used to provide additional information about the element.