What are Time Zones?
Time Zones are a geographical world globe division of 15o each, starting at Greenwich, in England, created to help people know what time is it now in another part of the world.
Nowadays it is much more a political division than geographical, because sometimes people needs to have the same time as other people in not-so-far locations. And for energy savings reasons, we have today the Daylight Savings Time, that are also a Time Zone variation.
Time Zones are usually defined by your country government or some astronomical institute, and is represented by 3 or 4 letters. See Section 3.2 for examples.