how customize tag cloud widget

I would customize the “Tag Cloud” widget layout. I checked the page where the widget was shown and I saw the following code:

<span>
<span class=”icon”><i class=”icon-tag”></i>
</span>
<span class=”tag-name”>tag_post_category</span><span class=”counter” style=”margin-left: 10px;”>2</span></span>

but I haven’t found the php file where this code is located. Could you help me?

Regards.