<p>It doesn't look like basic javascript but nor can I use JQuery commands like <code>$('myId')</code>. Is this or similar functions documented anywhere?</p>

<p>For reason I don't want to go into, I am not able to use 3rd party libraries like JQuery but if some powerful javascript extensions come with asp then I would like to know about them. </p>

$get is a function from the (now depreciated) ms ajax core javascript library. In the future they will be using jquery therefore $get will just be $('#myid') so I dont understand your feelings about not using jQuery, MS have decided to embrace OpenSource and bundle it with visual studio, check out Scott Gu and Hanslemans announcements on their blogs.
The ms ajax $get('foo') and jQuery $('#someID') are both just shortened wrappers around native javascript document.getElementById('bla')
ms ajax docs
http://www.asp.net/ajax/documentation/