A time zone is a segment of the Earth (from the North Pole to the South Pole) normally 15 degrees wide, in which the same time is used. The actual time of any location in the time zone may vary.
Coordinated Universal Time (UTC) is used, based on Greenwich Mean Time.
The process of creating time zones arose in part from the spread of railways and the timetables necessary for their smooth running. (In the UK this could lead to two different times being in operation - local time and railway time, which eventually won out).
The boundaries can vary from the longitude to some extent, mainly to keep the whole of a nation, state, or geographical subdivision within one time zone.
Daylight Saving Time
A list of UTC time zones, on Wikipedia can be found at