double degree - straight science takes 3 years and straight engineering takes 4 years, but you can combine the two (since some courses in engineering degree are from science faculty) into a 5 year combined or 'double' degree
major, double major - major is your major area of study. you can do a Bach of commerce with a major in Accounting, or finance, or ...
double major is when you do two major areas of study (B commerce in Accounting and Finance)
minor - normally in a straight commerce you can do a Major and a Minor, OR two majors. I think in unsw commerce if you do a double major you do 7 courses of each major, but if you chose to do one major (and therefor one minor) you do 8 courses of the major and 4 of the minor (and pick up 2 more electives)
post graduate and undergraduate
post graduate course is a course thats targetted to people that have graduated from an undergraduate
undergrad courses is simply that you would do straight out of school (ie bachelors of whatever), and postgrad courses are masters (research or coursework) and doctorates
at unsw almost as many students are doing coursework masters as all the undergrad bachelors
