JQuery :: Initiate Ajax Get When User Fills Textbox And Clicks Off

I've been reading about jquery and ajax and I'm not exactly 100% sure as to how to implement it. Essentially I have the following:
A textbox
A select box
Another select box

As the user fills data in the textbox and then clicks off I want the javascript to fire, submit the value in the textbox to a url (I can handle the backend side of it all fine) which will then send new data to the select boxes. Similarly for the second select box when a user clicks off (After selecting) I need the jquery to fire again. I've seen this code and think I get how to write it in, but I'm not sure how to do the (When user clicks off bit of it).
$.get('ajax/test.html', function(data) {
$('.result').html(data);
alert('Load was performed.');
});