$.ajax – dataType

• contentType is the HTTP header sent to the server, specifying a particular format.
  Example: I’m sending JSON or XML
• dataType is you telling jQuery what kind of response to expect.
  Expecting JSON, or XML, or HTML, etc. The default is for jQuery to try and figure it out.

The $.ajax() documentation has full descriptions of these as well.

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In your particular case, the first is asking for the response to be in UTF-8, the second doesn’t care. Also the first is treating the response as a JavaScript object, the second is going to treat it as a string.

So the first would be:

success: function(data) {
  // get data, e.g. data.title;
}

The second:

success: function(data) {
  alert("Here's lots of data, just a string: " + data);
}