To answer your "What is it?" question... The simplest way to descrive it is that ISDN is a digital version of the Public Switched Telephone Network (analogue phone system that your home phoen is plugged into). Every ISDN line has its own phone number and the terminal adapter (like a modem but it doesn't modulate and demodulate since the line is digital) simply calls another ISDN line to establish the call. You can even get ISDN phones. It stands for Integrated Services Digital Network.
I personally wouldn't use ISDN. Looking at Telstra's web page for OnRamp pricing it seems too expensive for what you get.
http://www.telstra.com.au/onramp/pricing.htm
I'd go with either ADSL or Cable (or BroadBand or whatever they call it nowadays...), but if you can't that that in your area then ISDN might be the way to go. Or maybe Satellite access. It's like Cable but without the... cable!
One good thing about ISDN is that if you have a friend who also has ISDN you can call them and connect directly to each other, bypassing the Internet. Good for stuff like LAN games where you need an opponent but don't need net access.
But then again, I'm a tight bugger and I dial into work for my net access... hehehehehehe