The documentation is always a good place to start, for example, the material you find in the ui object for draggable is available here: http://jqueryui.com/demos/draggable/ . An event object is always a source event, and a ui object contains information added by jQuery UI.
If you want, you can also do your own digging. Using console.log in Firefox with Firebug and Firequery add-ons, you can look at the insides of both objects. For example, using this code:
$('#test').draggable({ start: function(event, ui){ console.log(event); console.log(ui); } });
The ui object looks like this:

Yi Jiang Sep 11 '10 at 16:52 2010-09-11 16:52
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