Because of the eastward motion of the sun, the sunrise and the sunset times are different for countries located in the different geographic locations. Hence the time differs from country to country. Geographically the world is divided into various sectors called Time Zones, essentially based on the sunrise and sunset times. Some countries like Australia, USA and Russia are so large that they have two or more Time Zones for the different parts of the country. The fixed point or the parameter for judging the time zones is 00.00 degrees eastern longitude, which is called the 00 time zone. As it goes over Greenwich in England it is called Greenwich Mean Time, or GMT in short. Countries to the east of Greenwich are ahead in time vis-à-vis GMT and the countries to the west of it are behind GMT. So Greenwich to the east till the International Dateline, which is 180 degrees eastern longitude, the time is +GMT and Greenwich to the west till the International dateline is -GMT. As we know India is +5.30GMT and New York is -5GMT. So these are referred to as +5.30 Time Zone and – 5 Time Zone.
During the summer months, especially in the west, the days are long as the Sun rises early. Hence in several countries the clocks are advanced by one hour. This is known as Daylight Saving Time or DST in short. Normally it is from end March to end October, but varies from country to country.
The difference in time zones and the daylight saving time, are very important ones for astrologers while calculating horoscopes.
The picture given below, gives a good idea of the time zones of the world. The multi colored times given just below the clocks show the actual time for each region and you can see the difference. Please note as the DST is in operation, the New York to India difference in 11.30 hrs, instead of 10.30 hrs. The red and green bars at the bottom show the actual time zones.
If you want to see a bigger picture showing the countries and places, or want to see the list of countries observing the DST, you will find two links below the picture.