I am a newbie at PHP; today I see some code as the following in Magento `top.phtml`.

```
<?php $_menu = $this->renderCategoriesMenuHtml(0,'level-top') ?>
<?php if($_menu): ?>
<div class="nav-container">
    <ul id="nav">
        <!--NEW HOME LINK -->
        <li class="home"><a href="<?php echo $this->getUrl('') ?>"><?php echo $this->__('Home') ?></a>"</li>
        <!--NEW HOME LINK -->
        <?php echo $_menu ?>
    </ul>
</div>
<?php endif ?>

```

I know `$this` is the self of the `class`, it's only used in the class to refer to the method or property, on the above code, there is no class has been defined, why it can use $this keyword directly? What does `$this->__('Home')` stand for?