Extensible Markup Language (XML) is a structured document format that defines human- and machine-readable encoding rules.
Extensible Markup Language
[Wikipedia] defines XML as follows:
XML (Extensible Markup Language) is a set of rules for encoding documents in both human-readable and machine-readable form. It is defined in the XML 1.0 Specification produced by the [W3C], and several other related specifications, all gratis open standards.
In short, XML is:
- designed to transport and store data; and
- a flexible text format derived from SGML (ISO 8879).
The design goals of XML emphasize simplicity, generality, and usability over the Internet. It is a textual data format with strong support via Unicode for the languages of the world. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.
Many of salesforce's web services communicate using XML. The SOAP API uses XML for its message format.