Time zone (or timezone) is an area on the world of a defined time offset from UTC.
The need for time zones came as railroad development allowed people to travel longer distances.
Because of the earth’s orbit and tilt, day and night vary around the world, so to compensate for that, each city kept its own time.
This made it hard to synchronize between rail stations in different cities, so time zones were invented to allow larger areas to have a standard time.
UTC (Coordinated Universal Time) is a time reference for all time zones and is not subjected to daylight saving time (DST) changes.
Offsets describe the time difference from UTC. Most of the time zones have offsets of whole number of hours, but some offsets also include minutes (Asia/Kathmandu is +05:45).
Offsets are written in the format ±hh:mm or ±hh.
Most of the time zones are named after a main city in the area, (e.g. America/New_York, Europe/Berlin, Asia/Singapore) but there are also more general time zones (like PST, EST, CST) that cover larger areas.
These general time zones may be ambiguous (IST can be Israel, Indian or Irish Standard Time) and therefore discouraged.
Another reason not to use them is that some places in the same general time zone have different DST times.
It is therefore better to use the more specific time zones for each country/state.
In some places during the summer, the sun rises and sets earlier.
In order to have more hours of light, some countries advance the clock (usually by 1 hour) in what is called DST.
This also bring a lot of mess to the already messy time zones issue, so a few countries cancelled their DST shifts, but it still remains in many others.