What is the difference between AJAX, RESTful/Rest, JSON and JSONP?

• Ajax - "Asynchronous Javascript and XML". Ajax loosely defines a set of technologies to help make web applications present a richer user experience. Data updating and refreshing of the screen is done asynchronously using javascript and xml (or json or just a normal http post).

• JSON - "Javascript Object Notation". JSON is like xml in that it can be used to describe objects, but it's more compact and has the advantage of being actual javascript. An object expressed in JSON can be converted into an actual object to be manipulated in javascript code.

• By default, Ajax requests have to occur in the same domain of the page where the request originates. JSONP - "JSON with padding" - was created to allow you to request JSON resources from a different domain. (CORS is a newer and better alternative to JSONP.)

• REST - "Representational State Transfer". Applications using REST principles have a Url structure and a request/response pattern that revolve around the use of resources. In a pure model, the HTTP Verbs Get, Post, Put and Delete are used to retrieve, create, update and delete resources respectively. Put and Delete are often not used, leaving Get and Post to map to select (GET) and create, update and delete (POST)

Ajax, or more properly, AJAX, stands for Asynchronous Javascript And Xml. Technically it refers to any asynchronous request made by the browser (anything that uses an XmlHttpRequest) on behalf of some script running on the current page, regardless of what content-type is returned. It can also be used to describe a certain pattern of constructing a page/site where most/all of the content is fetched/updated dynamically on the page. When used to describe a data format, "ajax" typically means "xml".

JSON is a data encoding format. The name itself is an acronym for "JavaScript Object Notation". JSON-formatted data looks like:

{"key": "value1", "key2": {"number": 1, "array": [0, 1, 2]}}

JSON data may be fetched by an AJAX request, though it is quite commonly used in other contexts as a lightweight, extensible, and easy-to-parse data exchange format.

JSONP is simply JSON-formatted data wrapped in a callback function. The "P" stands for "with Padding", which is kind of stupid unless you like to think of function calls as "padding". In any case, JSONP data will look like:

someFunction({"key": "value1", "key2": {"number": 1, "array": [0, 1, 2]}});

As such, JSONP is really just a JavaScript snippet, and unlike JSON is not used outside of the context of JavaScript, browsers (or other JavaScript-capable clients), and AJAX requests. The reason for using JSONP is that it allows the same-origin policy to be subverted. A script that was sourced in from site X cannot make a direct request to site Y if site Y is on a different domain from site X. But if site Y's server can send JSONP-formatted responses, then the script from site X can add a new <script> tag to the document that references a URL on site Y, and when the response from site Y is loaded it will invoke some callback function that script X has defined in the document, thus giving script X access to data that was loaded dynamically from site Y.

Note that JSONP data is not (typically) requested using an XmlHttpRequest. It can be done this way, subject to the standard caveats of the same-origin policy, but then you lose the cross-domain magic that makes JSONP useful in the first place.

REST is simply the formal spec/description of how HTTP actually works/is intended to be used. If you understand the concept of a URL being used to request a corresponding resource from a server and the difference between Get and Post then you really know all you need to about REST.

Ajax stand for Asynchronous JavaScript and Xml/XhttpRequet (the X depends and changed since mostly json is used today.

It's a way to execute a request from the page using javascript to the server and receive some response. This response can be anything, json, xml, text, html, etc...

This makes pages look very responsive without having to reload the complete page to execute actions on it. For example posting this answer to your questions. :-)

Json is a data format stands for JavaScrip Object Notation. It's a lighter serialization format than xml and has the advantage to be JavaScript.

JsonP is the next and logical step to using Ajax with Json.

A server will response with JSONP wrapping the Json object in a callback function. The name of the function is passed by the client to the server, usually as a parameter in the querystring.The P stands for padding, since the server surrounds the json object with the name of the function and the object as an argument.

callback({"name":"my name"});

See: http://en.wikipedia.org/wiki/JSONP for a more detailed explanation.