the file _view.php is rendered by the index.php view trough the widget CListView. Open the file and you will see it.
So this widget call the code of _view.php foreach item of $dataProvider and pass a variable called $data that is an object with variables of Model "Post". The Model "Post" (open the /protected/models/post.php) has a function called getUrl
According of setter and getter of Yii, each function with name getVar1 or setVar1 is accessed by "Var1".
So the echo CHtml::link('Permalink', $data->url);
is the same with
echo CHtml::link('Permalink', $data->getUrl());