XML: What it is

Well-defined syntactic rules for marking up Unicode text, defining a hierarchy of elements with attributes.
Official recommendation of W3C (and working through ISO)
Agnostic as to:
platform, application, API, vendor
I18N-enabled
Suitable for complex and recursive structures
Human-readable
Parsing is trivial to implement
Validation is easy to implement, and easy to locate code for
Companion technologies and standards developing to flesh out the story