.js == A Javascript file. This is an actual API script, and the only thing you really care about at this point. Copy the source into a tab on your API page and game on! .json == A Javascript Object Notation file. This is a portable data interchange format (designed by my JS hero: Douglas Crockford!). It's a way to encode data in a machine readable format, but in this case is also very human readable. The package.json file describes the API script in a way that it is possible to write a system to interrelate all the API scripts. Roll20 Devs plan to use this information to provide an automated Script installation system later on down the road. At this point, you can completely ignore it (unless you're writing scripts). =D
