To make it as short and sweet as I can.
IPSC power factor = bullet weight x speed / 1000
production division, power factor 125 = minor
standard division, power factor 125 = minor and 170 = major
classic division, power factor 125= minor and 170 = major
open division, power factor 125 = minor and 160= major
3 scoring zones on target, A, C and D
minor is scored A=5, C=3 and D=1
major is scored A=5, C=4 and D=2
Production division, first shot must be double action 5+ lbs ( basically stock 9mm's da/sa and stryker)
Standard Division, first shot can be single action. (mostly .40's SA with some mods allowed)
Classic division, must be 1911's, ( 9mm minor 10 round mags or .40 and larger major 8 round mags)
Open division, kinda speaks for itself.
For a more detailed explanation.
http://www.ipsc.org/rules/divlistP.php