Confused between ajax and javascript

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Javascript

Ajax

Google-Maps

Hi,

I am getting kind of confused as I was coding a simple input text box that takes in
a zip code, which then displays a google map with its map api.

I initially started out with ajax in mind, but as i was doing it, I found
that i ended up with a javascript function that takes in the zip code from my form.
The function just initializes the map element from the zip code input, where I can just display it on the page.

My text box has an onkeyup parameter that calls the function as below:

function initialize(str) {
  if (GBrowserIsCompatible()) {
    var map = new GMap2(document.getElementById("map_canvas"));

    var geocoder = new GClientGeocoder();
    geocoder.setBaseCountryCode("us");

    geocoder.getLatLng(
      str,
      function(point) {
        if (point) {
          map.setCenter(point, 13);
          var marker = new GMarker(point);
          map.addOverlay(marker);
        }
      }
    );
  }
}

I'm probably not that strong in my concepts, so what's this document.getElementById part? Is that a javascript thing?

So why do we need ajax then? For cases where javascript is not supported?
Confused.


Brian