Basically, XML document is a collection of data. Though by collection of data, XML document is said as database in other sense a mere collection of data doesn't make much difference between the other files that store data. After all, all files contain data of some sort.
Still as a "database" format, XML has some advantages than other forms of data collection. XML in a database format is a self describing, portable, and can describe data in tree or graph structure. It is self-describing in the sense that the markup describes the structure and type names of the data, although not the semantics, it is portable means it uses Unicode, and it can describe data in tree or graph structures.
XML is treated as database because of its surrounding capabilities like: storage (XML documents), schemas (DTDs, XML Schemas, RELAX NG, and so on), query languages (XQuery, XPath, XQL, XML-QL, QUILT, etc.), programming interfaces (SAX, DOM, JDOM), like other databases. Hanson expresses that "XML data are extremely well-suited to hierarchical storage,"
But long winded nature and the slow accessibility to data due to parsing and text conversion are the drawbacks of this technology. ...