Here in NSW we all undertake in our final year of High School a number of different subjects (the only compulsory subject being English). Each of these subjects is given a certain "Unit" value, most often 2 but sometimes up to 4 depending on the difficulty and scope of the subject. The final mark given is called the Universities Admissions Index (UAI).
A maximum of 14 units can be studied in the final year with assessment based on assignments and internal exams (25%), mid-year internal exams (trials, 25%) and final examinations set by the NSW Board of Studies (the dreaded HSC, 50%). Exams are only taken in the subjects you choose, everybody does the two English exams but then you only get examined on the individual subjects you chose to study. There are a number of subjects that count towards a final Year 12 mark but not towards the UAI. These subjects are often the practical subjects and are used by school-leavers who most likely go into a trade of some sort, where the specific mark in certain subjects are vastly more important than the overall mark. However seeing as most school-leavers (we don't really call the graduates, you graduate from university, not High School and the whole thing is very low key, usually just a nice year-group dinner after a muck up day. Certainly no handing out of diplomas or caps and gowns or big school parties), go into a trade after year 10, these subjects are not as popular.
These results for the individual subjects (best 10 units) are then normalised against certain standards and then scaled according to the difficulty of each individual's chosen subject and how they performed in said subject compared to the rest of their classmates.
This final result is given out of 100 and represents the percentage of students who had a lower mark than yours. Because of the system there are usually about 10 who get 100 and it goes down from there. You get the final mark by either SMS text message to your mobile phone, on the internet or a week later in the mail.
I got 96.00 for my HSC back in 2000 which meant that my score was higher than 96% of the 65,000 students undertaking the exams that year.
The mark is used by universities to fill places in the courses the students have pre-selected online. The universities start filling places from the highest mark down until the places are full - the lowest mark in the course is given as the "cut-off mark" but is not actually a measure of the difficulty of the course, rather it shows the course's popularity.
Here we can go into medicine and law straight out of high school (they are given as part of a double degree, so law students would graduate after 5.5 years with an LLB and a BSomethingelse, Med students often graduate with an MBBS and a BA or a BSc). We don't have to do a big application for any universities as they select students (for most courses, Creative Arts is often different) based purely on their marks. So for those wanting to go to uni right after high school - a reasonable number but quite a lot take the Gap Year off and go travelling - the HSC results are the most important results.