What are .tpl files? PHP, web design

Someone wants me to redesign a site run in PHP (VideoCMS). But when I asked him to send me the source he has given me *.tpl files instead of *.php. There is some code inside them:

{include file='header.tpl' p="article"}

<br />
<table width="886" border="0" cellspacing="0" cellpadding="0">
  <tr>
    <td width="150" valign="top">
    <div id="reg_box">
    <h3 class="captions">{$lang.articles}</h3>
        <div id="list_cats">
        <ul>
            {$article_categories}
        </ul>
        </div>
    </div>
    <br />
    <div id="reg_box">
    <h3 class="captions">{$lang.members}</h3>
    {if $logged_in == '1'}
    {include file='loggedin_body.tpl'}
    {else}
    {include file='login_body.tpl'}
    {/if}

or

{include file='header.tpl' p="index"}

{php} $_SESSION['isFair'] = "Yes"; {/php}

What's the interpreter of the code? How can I redesign this site?