How can I get the content of CKEditor using JQuery?

use the CKEditor.editor.getData() call on the instance. That is to say:

HTML

<textarea id="my-editor">
<input id="send" type="button" value="Send">

JS for CKEditor 4.0.x

$('#send').click(function() {
    var value = CKEDITOR.instances['DOM-ID-HERE'].getData()
    // send your ajax request with value
    // profit!
});

JS for CKEditor 3.6.x

var editor = CKEDITOR.editor.replace('my-editor');

$('#send').click(function() {
    var value = editor.getData();
    // send your ajax request with value
    // profit!
});