Explosion Hazardous Areas are divided into zones according to risk probabilities. Within the European Union, these are defined by IEC standards (IEC60079-10-1 or 2) as directed by ATEX. There are three zones for gas, Zones 0-2, and three for dust, Zones 20-22. Put simply, the 0-zones (0 and 20) are highest risk probability, and 2-zones lowest (2 and 22).
How the risks are classified is a bit more complex. One Hazardous Area can have several different zones. Risk factors include how long the explosion hazard is present and how far the source of the hazard is from the area in question.
With the zone classifications, there is a big difference between Zones 0-1 and Zone 2. The demands on Ex-rated products for Zone 2 are significantly lower. They are so low that you could almost say Zone 2 products are unregulated. There is no 3rd party testing, for example.