XML documents are structured text files that use a markup language to store and transport data. They consist of elements, which are defined by tags, and can include attributes to provide additional information. This format is both human-readable and machine-readable, making it useful for data interchange between different systems. The main purpose of XML is to facilitate the sharing of structured information across diverse platforms and applications. It is widely used in web services, configuration files, and data storage, allowing for easy data manipulation and retrieval while maintaining a clear hierarchy and organization.