Have a look at JQuery. It's very easy to use.
You will need to put some effort in, and be prepared to learn.
What it does is, for example, you set up the fields on a form to trigger the Jquery in the background (onclick/onfocus etc), it HTTP POSTS (or GETS, depends on how you configure it) the data to the server and then the reply is sent back to Jquery where it is then placed onto the page inside an element which you specify.
I use this to tell users whether a username is already in use immediately after they have typed it in.
http://jquery.com/