There are 24 times zones in the world just as there are 24 hours in a day. Time zones are divided by longitude. The reason for time zones is to allow for the natural rising and setting of the sun.
Each time zone covers approximately 15 degrees of the planet's surface and represents a single hour of the time it takes for the earth to rotate on its axis. Greenwich, England, is the point from which time zones are measured. Each zone east of Greenwich is an hour forward and each zone west of Greenwich is an hour behind. This accounts for the origin of the term "Greenwich Mean Time."