There are three issues with gassoline: Octane, additives, and quality. Quality refers to how much sediment and water is present. Pretty much these days, all octane ratings have the same additives.
Calulating octane is a pretty complex chemical equation. But what is generally means is how much potential energy the fuel has. Higher octane fuels need more compression before they will ignite and then you get more work or energy from them.
Low octane gas in a high compression motor will cause knock. This is when the fuel ignites prior to spark. Moden cars have knock sensors that I think will advance spark if knock is sensed so that the engine is not damaged. You will loose power.
Using high octane gas in a car designed for low octane (lower compression) I believe will result in fuel not being burned, thus leaving depsosits.
Check out this link too for more info:
http://auto.howstuffworks.com/question90.htm