The type of data you expect to get back from an Ajax request in jQuery generally requires some instruction.  By the method name the data type is specified and in different cases it is provided as part of a configuration object. There are various options:

Text

For transferring  simple strings on different pages

Example

$.ajax({
    type: "POST",
    url:"abc.php",
    data: "{}",
    async: true,
    dataType: "text",
    success: function( data ) {

        console.log(data);

    }
}); 

Html

For transferring blocks of HTML to be placed on the page.

 $.ajax({
        type : 'POST',
        url : 'post.php',
        dataType : 'json',
        data: {
            email : $('#email').val()
        },
        

Script

For adding a new script to the page.

 $.ajax({ url: 'http://unknown.jquery.com/foo', dataType: 'script', cache: true })
  .then(function () { console.log( 'done' ); }, function () { console.log( 'fail' ); });

Json

For transferring JSON-formatted data, which can include strings, arrays, and objects.

$.ajax({
    url: "text.php",
    type: "POST",
    data: {
        amount: amount,
        firstName: firstName,
        lastName: lastName,
        email: email
    },
    dataType: "JSON",
    success: function (data) {
        console.log("ok");
        $("#result").text(data);
    }
});

Jsonp

For transferring JSON data from another domain.

$.ajax({
   type: 'GET',
    url: url,
    async: false,
    jsonpCallback: 'jsonCallback',
    contentType: "application/json",
    dataType: 'jsonp',
    success: function(json) {
       console.dir(json.sites);
    },
    error: function(e) {
       console.log(e.message);
    }
});

XML

For transferring data in a custom XML schema.

 $.ajax({
        url: 'http://theresidency.libsyn.com/rss',
        type: 'GET',
        dataType: "xml",
        success: parseXml
    });
});