I've often heard people remark that they "only" use hi-octane gas for their car. When I ask why, I get a lot of different answers but rarely do they have any basis in reality ;') The octane rating of gasoline simply rates how effectively the fuel resists detonation. Or, in wiki-words:

The octane rating is a measure of the autoignition resistance of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines.

For reference, 'autoignition'='detonation'. It might help to think of detonation as the fuel-mix 'exploding' rather then 'burning'. Detonation can occur for several reasons, but is limited to when an engine is under full load (best example is 50~70mph up a relatively steep hill, with engine warmed up). If detonation does occur it sounds very much like marbles rattling around inside two tin pie pans.

The problem with detonation is that it places an enormous strain on the engine components. Not a good thing. Bad. Quick solution is to pull your foot up off the gas, decrease the 'load' on the engine. I recommend using regular gas (about 87 octane). If, and only if you experience detonation, then use the next highest octane level gas (usually 89 or 90 octane). If you still experience detonation, then try the highest usually available (92 octane). Higher octane gasoline will NOT make your engine run cleaner. Ever. Higher octane gasoline will NOT make your engine last longer. Unless you are experiencing detonation.

This Public Service Announcement brought to you by an ex-mechanic of exotic autos, motorcycles.