They are rated by a formula that defines the blends in the fuels. from 87 to 89 to 91 to 93 octane. They vary in many areas even different states and can depend on "Re Formulated" fuels for areas not meeting air quality standards.
It can all get very complicated but bottom line is just find the best "Branded" fuel with all of the additives that may or may not be included in the "bargan" brands. They cost a little more but belive me, you get what you pay for.
The higher the octane the less chance of engine knock that tells the ECM to retard the timming and thus lowers performance. "Pinging or engine knock is bad, it can damage your engine but most engine designs today have the means to compensate for low octane.
Hope this helps
2010 2SS/RS A6 L99 Born 09/16/2009 Vin#9146985
ModernCamaro Car of the Month Jan 2013
CAI,Axle back exhaust, V Max Throttle Body