As other have said, super petrol is about octane - but the important thing to remember about octane is that you need to have enough, but having more doesn't help. So an engine designed for 95 will run badly on anything less, but won't run any better on 98.
With diesel it's about cleanliness - so I don't use supermarket diesel in our cars and every 4 refills I'll put in a tank of super if the price is reasonable.