Json is a textual "lightweight data-interchange format", based on a subset of Javascript, and it is often used within ajax based web applications (e.g. by means of the JSON-RPC-Java library).
Curiosity can parse Json Web Sources and transform them in Xml: this way you can acquire the data therein expressed.
<webSource name="aJsonSource"
content="json">
...
</webSource>
Please, try loading a Json Web Source or a Json url in Curiosity Studio (remember to explicitly select Json content if the url doesn't end with the .js extension) in order to clearly understand how the document is tranformed.