A time zone or timezone is a region of the earth that has uniform
standard time, usually referred to as the local time. By convention,
time zones compute their local time as an offset from the Coordinated
Universal Time, UTC ( or Greenwich Mean Time, GMT). Local time is
UTC plus the current time zone offset for the considered location.
GMT is a term originally referring to mean solar time at the Royal
Observatory in Greenwich, London where sits on the longitude 0°.
It is commonly used in practice to refer to UTC when this is viewed
as a time zone.
Moreover, Time zones are divided into standard and daylight saving
(or summer). Daylight saving time zones (or summer time zones) include
an offset (typically +1) for daylight saving time.
If you have the basic knowledge of a SUNDIAL,
you will understand why Time Zones exist.