Using the XML HTTP Request object
Here s a really good primer on starting with all this AJAX stuff: Using the XML HTTP Request object.
It gets you up and running quickly with some basic example of running HTTP requests (the “Right Way(tm)”), including JSON and SOAP.
The “Right Way” being with the browser’s XMLHTTPRequest object, which you’re going to need a recent Internet Explorer, Mozilla (1.6 and all Firefox versions), Opera (7.6), Safari (1.2) or Konqueror (3.4). As can be seen from the above version numbers, its not a huge hurdle – unless you were living under a rock in the last year you most likely have the minimum requirements (especially if you are still using Internet Explorer – having pioneered this technology, Microsoft’s Internet Explorer 4 features an XMLHTTP ActiveX).
The older method – as mentioned in the above referenced document – used iframes for remote HTTP calls. A simple (and very old) example that uses the iframe method can be found on my site here. Its mostly an example of client side sorting rather then the iframe data loading, but the javascript have everything, if not properly documented, then at least properly divided into nice functions.