Protocol Buffers: better than XML (?!)

From Google:
''
Protocol buffers are a flexible, efficient, automated mechanism for serializing structured data – think XML, but smaller, faster, and simpler.
...
You specify how you want the information you're serializing to be structured by defining protocol buffer message types in .proto files.
...
Once you've defined your messages, you run the protocol buffer compiler for your application's language on your .proto file to generate data access classes.
...
You can add new fields to your message formats without breaking backwards-compatibility; old binaries simply ignore the new field when parsing. So if you have a communications protocol that uses protocol buffers as its data format, you can extend your protocol without having to worry about breaking existing code.
''

Read here.