jdm just means 'japanese domestic market'. therefore, a jdm celica is just a celica that is sold in japan.
the differences between jdm and american spec celicas are numerious, but the most basic are steering wheel on the opposite side, revised front lip, lcd dash, climate control, retuned ecu.
to answer the gas octane rating question: japanese gas is usually much higher octane (up to 108) while american premium gas is about 93 octane. the higher octane gas means fuel burns more efficiently (broken down easier) and increases power slightly. to take advantage of the cleaner burning fuel, all jdm spec cars have different ecu programming to take advantage of this and the cars run better as well as more powerful