It depends what uni you go to.
My uni (as with most others) believe that 40 units is what you should be doing per semester. 40 units supposedly equates to 40 hours total effort per week (this includes class time, so if you have 15 hours in lectures/tutes/labs, you're expected to put in 25 hours to assignments/readings/study/etc). Generally under this system, you do 4 subjects per semester at 10 units each (thus, 10 hours total effort per subject per week), although sometimes it varies (I generally have to do 2x 10 units. 1x 20 units).
A major is the area in which you concentrate on. Each uni and each programme will set out how many credit points you have to complete in that discipline for it to be your major. For example, when I did science, say you wanted to major in biology, you had to do 20 units of it at 1000 level (1st yr), then a minimum of 40 units of it at 3000 level (3rd yr). It was upto you if you took all of these 40 units in first
sem, or 20 units per semester, or even took more than was required.
Double degrees, you do 2 degrees at once. I think they're generally one 'shorter' degree (say, commerce for 3 yrs) and one 'longer' one (law for 5 yrs)...but you mix it up...yeah, I don't really get it so I won't try and explain it any more.