The difference between 95RON and 97RON is that the 97 has a higher octane rating. In basic terms, it just means its a higher quality fuel, and has a higher chance of only combusting when its meant to combust. The higher the octane of the fuel, the less fuel has to be pumped in, resulting in leaner mixtures, more air per cycle and thus more powah.
In my opinion, there is no difference between 95 and 97 for a close-to-standard street car. It is only a real issue when you have an aftermarket ECU of some sort and have tampered with the stock fuel maps to suit 97 to take full advantage of it, leaning the A/F ratios out a bit to squeeze out every last drop of power.
In that situation, using a lower octane fuel will mean a higher chance of detonation or knocking, which is basically the fuel combusting prematurely, before the air is compressed to its ideal point. For a simplified example, imagine an individual piston on its way upwards, compressing the air/fuel mixture, and at about 75% of its way up, the fuel combusts, pushing the piston back down again, while the crankshaft is still pushing the piston upwards and you can begin to see how bad it is for your engine. It is just something you want to avoid if you value your engine's health :D
