It is a very common data format used for asynchronous browser–server communication, including as a replacement for XMLin some AJAX-style systems.
Invented by Douglas Crockford.
This is an almost complete copy from the English Wikipedia article (except for messing up some whitespacing) in violation of its copyright license. There would be things to say about JSONs role in hypertext systems, but this is not the place for it.
Yes, it’s a bit of a placeholder. 🙂