JSON is simply a useful way to send and receive strings that represent objects in JavaScript. In these days I'm working on an application that uses JSON strings to store and retrieve data, and I found a very simple way to get data from JSON strings, that is jQuery. This library has a jQuery.getJSON() method, which let you to load JSON-encoded data from the server (or locally) using a GET HTTP request. Here you can find all the details you need to use this method.
Obviously you could choose not to use any third-part library and do what you need in vanilla JavaScript, but jQuery is very useful since it helps to avoid common cross-browser issues.
In my application I store data from a JSON string in this way:
var placesList;
jQuery.getJSON("places.txt").done(function (data) {
  placesList = data;
});
that is using an external variable to store them using an anonymous function. As you can see, my URL here is places.txt, but you can use any valid URL you want that provide a JSON string.