Temperature Classes

It's all to do with gas group. T1 will cover hydrogen and little else. T4 will cover most things except carbon disulphide and one or two other specials.

The T class is simply the maximum surface temperature that can be expected under fault conditions. It must always be below the autoignition temperature of the gas(es) present.

The article in Wikipedia gives more details.

Do not attempt to select, install, commission or modify hazardous area electrical circuits without having received beforehand suitable and adequate training.

Good answer has been given by PWSlack. I just would like to add more:

The temperature class defined in North America (NEC) and Europe (IEC) are:

The temperature class of the explosion-proof equipment ensures that the surface temperature of the equipment will never exceed this value, even in the worst condition (such as the motor burns, explosion occurs in a box etc). The lower is the temperature class, the higher is the price of the equipment. When selecting the equipment, make sure temperature class value remains almost 80% of the ignition temperature of the LEL (Lower Explosive Limit) of the surrounding gases. By this selection, it is guaranteed that there will not be any fire for the worst condition of the equipment.

- MS