Zend: Where to define the layout includes

Hi,

I am just learning the zend framework and i know this is a very basic question. I have created a layout and it is working fine. My layout is something like following

<html>
<head>

</head>
<body>
<?php echo $this->layout()->header; ?>

<?php echo $this->layout()->content; ?>

<?php echo $this->layout()->footer; ?>
</body>

</html>

my question is where will i define $this->layout()->header? is this a simple phtml file or do i need to write any class? Also if it is a simple phtml file where do i store it? In the same directory of layout file or any other directory?