I’ve recently used it like this:
<i class="icon-ok-sign" rel="tooltip" title="Key active" id="blah"></i>
Which produces:

You set the positioning of the tooltip and other features(delays etc) by declaring it using js:
$(document).ready(function(){
$("[rel=tooltip]").tooltip({ placement: 'right'});
});
As mentioned in comments, you can find other attributes/options here.