major and minor are what is called Power Factors the IPSC measures your bullet. It if bullet wieght times velocity divided by 1000. For example 180gr bullet x 1000 FPS =180,000/1000 so your PF is 180. For major you need 175 and most people go for 180 so they have a slight buffer for varying conditions, chronos etc. The advantage to shooting major is that if you drop a shot out of the A zone you get more points than minor.
Major
A = 5
B&C = 4
D = 2
Minor
A = 5
B&C = 3
D = 1
Matt