The closer the figure is to zero, the better. So in the figures you've given above, -75 is better than -100.
In my experience -75 provide a reasonable service. -85 provides an OK service. -100 provides a barely usable service. -110 provides no service at all.
This figure represents your signal strength to the cellular tower. The closer you are to a tower, the closer to zero the number will be.
Note that although I mentioned above that -75 provides a reasonable service, keep in mind that if the tower you're connected to is heavily congested the service won't be all that great. It is similar to the way in which ADSL works - even if you sync at 24mb/1mb, if the DSLAM is heavily congested, you'll get poor throughput.
Is this interpretation the same when talking about 2G or 3G?
By the way, if you're using 3G and Telstra is your ISP, take a look at this page. You can force the modem to connect to the 850mhz frequency which performs much better than the 2100mhz frequency.