Yeah basically what they said. So when you get your results back, it should have two important numbers on there. The first one is a raw mark out of 100 (after scaling), while the second one is a number between 1 and 7 for your GPA.
The first mark is used to calculate your second mark for your GPA, but also to calculate your WAM. Here's an example. This is assuming you only had one assignment out of 100 in each unit in one semester. Unlucky....
First assignment
70/100 (First mark) 5 (second mark)
Second Assignment
50/100 (First mark) 4 (second mark)
Third Assignment
(90/100) (First mark) 7 (second mark)
So to calculate your WAM, your first, raw mark in each assignment is taken and added together, and then divided by the number of units. In this case 70+50+90 divided by 3. That gives you a WAM of 70. The second mark - the one with the single digit number - is also given the same process, in this case giving you a GPA of 5.33. Your WAM is a calculation of your raw mark in each piece of assessment, which gives the average mark you get across the board. The GPA is a broader, less detailed version of your performance.
For me for example, I have a GPA of 5.75 and a WAM of 76.13. Which blows. Based on the WAM, I should have a GPA of 6.00 giving me a distinction average (4 being a pass, 5 credit, 6 distinction, 7 HD). But due to me missing out on a HD in one of my units by a measly one mark because of useless group members, I missed that GPA of 6.00. How I came up with that is that your GPA is based on that raw mark i.e:
4 (Pass) - 50-64
5 (Credit) - 65-74
6 (Distinction) - 75-84
7 (HD) - 85+
Does that make sense?
As to your second question, it depends on the university. Some use your GPA, some use your WAM, some like to see well how you can grovel (portfolio and personal statements). Really does depend. I'm going for a transfer, and I am a little bemused with the whole process. Wish it was standardised across each uni for most courses. I can understand some not requiring or requiring a portfolio. But others...