Difference between $.post and $.ajax?

The problem here is not the fact $.ajax() is not working, it is because you did not set the type parameter in the Ajax request and it defaults to a GET request. The data is sent via the query string for get and if your backend expects them as post parameters, it will not read them.

$.post is just a call with $.ajax(), just with the type set. Read the docs and you will see that $.ajax() defaults to a GET as I mentioned above.

If you go to the jQuery.post page in the jQuery docs it shows you the $.ajax request with the type set. Again read the docs.

This jquery forum thread sums it up:

$.post is a shorthand way of using $.ajax for POST requests, so there isn't a great deal of difference between using the two - they are both made possible using the same underlying code. $.get works on a similar principle.

—addyosmani

In short, this:

$.post( "/ajax", {"data" : json }) 

Is equivalent to the following:

$.ajax({ 
  type: "POST", 
  url: "/ajax", 
  data: {"data": json} 
});