It is a method for packing up electronic data and documents so that having been packed up on one computer anywhere in the world, they can be reliably unpacked on any other computer (running any combination of software and hardware) anywhere else in the world, then or many years later, and used for whatever purpose desired, without regard to the originator’s intent.
Grrr. I mentioned before that hpricot downcases element names when parsing HTML, but it does when parsing XML as well. It doesn't claim to be a conforming parser, but it makes it a little less useful.