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lets clear things up:
Octane is a hydrocarbon. It is a major component of petrol. well actually the stuff in petrol is 2,2,4 methylpentane, this is a structural isomer of octane as it has the same molecular formula)

Octane is a longer molecule than heptane which has 7 carbon atoms as opposed to octane which has 8. The longer the molecule the higher the dispersion forces and therefore the less volatile it is, therefore the higher its autoignition temperature, therefore increasing the burn time, therefore creating a more controlled burn, therefore reducing the chance of detonation.

Detonation is the collision of flame fronts within the cylinder. ie if one part of the charge autoignites then when the spark plug fires up the rest, there will be a collision of flame fronts.
and thats what causes engine damage and that awful awful noise of your hard work going down the drain....

Octane is measured relative to a mixture of isooctane and heptane. a 95 octane fuel has 95% (by volume) isooctane and 5% heptane.

however some fuels have higher than 100 octane. how is this?

Because octane is not the most autoignition resistant compund know (its far from it...) and some fuels (such as ethanol) have a higher measure RON and MON than 100% octane. meaning that it will burn slower and more controlled than 100% octane.

RON (Research Octane Number) and MON (Motor Octane Number) are the ways of measuring the way the fuel burns. so, if a fuel has 90RON then it has 90% octane

MON however is a more thorough way of testing the burning characteristics of a fuel, and the MON rating will always be lower than the MON rating.
Note:
Octane semistructurale formula:
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH3
molecular formula C8H18
2,2,4 trimethylpentane semistructural formula:
CH3 C3H6 CH2 C2H4 CH3
making a molecular formula of C8H18 again

pentane semistructural formula:
CH3CH2CH2CH2CH2CH2CH3
making a molecular formula:
C7H16