An XML file contains text formatted in XML, the Extensible Markup Language. It contains tags, which define the markup code, and content, which is placed between each set of tags.
XML is "extensible" because it supports user-defined elements. This means any type of information can be stored within an XML file. Since XML offers a wide range of flexibility, many programs use XML files to save data. While various programs may use XML files for different purposes, all XML files are saved in a plain text format, which means they can be opened in a basic text editor.