XSS/AJAX question - how do I send data?

[eluser]taewoo[/eluser]
[quote author="slowgary" date="1242468920"]
Change the domain, script and variables, but leave the "?callback=?" portion. In your server-side script you'd access the above example as $_GET['username'] and $_GET['userid']. Add as many parameters as you need.[/quote]

Thanks slowgary (though u don't sound very "slow")

What's the "callback" param for? Is that even necesary?