What is the difference between JSON and AJAX with jQuery?

The dataType option simply changes what type of data jquery should expect from the server. It can be json, jsonp, html, text, xml, or any custom datatype that you define a converter for. They all work in all browsers.

By default jQuery will try to detect what type of data is being returned if you do not supply a dataType option, however I find that it doesn't automatically detect very well.

Edit:

but what if i need to return an object? is basically the answer of a database consult... is it better to use json or only jquery?

You can return an object in the form of html, xml, json, or jsonp. As long as it is in one of those formats, jQuery will be able to interpret it.

I think you are confusing the terms.

AJAX stands for Asynchronous Javascript and XML, which is a mechanism used to launch asynchronous HTTP requests to a server using JavaScript. Don't let the name fool you; there's no restriction on you only retrieving JavaScript or XML from this technique. You can quite happily return other data formats as well (HTML, plain text and JSON, to list a few).

JSON is just one of these formats. It's a data interchange format, where-as AJAX is a technique to communicate with a server after the initate page load has completed.

To answer your question on whether you need to specify the dataType; jQuery will best guess the response format (be it HTML or JSON etc), so you're usually fine to omit it.