asynchronous Google DFP for Publishers Tag code with explanations

body section

<!-- the id must match the id defined in the slot -->
<!-- style attribut is optional, but useful to prevent to shift layout after ad is loaded -->
<div id="banner1" style="width:300px; height:250px;">
    <script type="text/javascript">
        // id 'banner1' is used here, again
        googletag.cmd.push(function() { googletag.display('banner1'); });
    </script>
</div>

head section

<script type="text/javascript">
    var googletag = googletag || {};
    googletag.cmd = googletag.cmd || [];
    (function() {
      var gads = document.createElement("script");
      gads.async = true;
      gads.type = "text/javascript";
      var useSSL = "https:" == document.location.protocol;
      gads.src = (useSSL ? "https:" : "http:") + "//www.googletagservices.com/tag/js/gpt.js";
      var node =document.getElementsByTagName("script")[0];
      node.parentNode.insertBefore(gads, node);
     })();
</script>

<script type="text/javascript">
    // js pushed like this will be added to the gdfp queue and processed asynchronously
    googletag.cmd.push(function() {
        // 6355419 - your individual network code you find in the admin panel
        // Travel/Europe/France/Paris - name of your target ad unit
        // [300, 250] - allowed sizes of the creatives (should be associated with the ad unit)
        //      can also have multiple values, e.g. [[300,250],[468,60]]
        // banner1 - unique id matching the banner loaded in the header with the one in the body
        //      even though the ids generated by GDFP might look complicated, its just their pattern, but not to identify the ad unit on their server
        googletag.defineSlot('/6355419/Travel/Europe/France/Paris',[300, 250], "banner1")
            .addService(googletag.pubads());
            // (optional) slot-level key-value targeting
            // can also have multiple values, e.g. ("category", ["games","tests"])
            .setTargeting("pagetype", "article")
        // (optional) page-level key-value targeting; will be inherited by all ad units
        googletag.pubads().setTargeting("topic","app");
        // (optional) include this line if all ads should be loaded in a single request (only disable for specific use cases)
        googletag.pubads().enableSingleRequest();
        // wenn, yes, you really want to get the ads defined above, right?
        googletag.enableServices();
    });
</script>