Time zones are set according to geographical locations of a region for the benefit of commerce, trade, and social aspects. When you travel from pacific standard time to eastern standard time there is an addition of 3 hours as they lie in different geographical locations.
For example- PST =13:00 hrs, then EST = 16:00 hrs ( PST+3hrs)
Time Zones are put in place as a standardiztion for time in certain areas. They usually break up areas like the boarders of countries but in some cases as in the case of the United States there are several time Zones in one country.
The Eastern Time Zone makes up the East coast and beyond to a certain point while the Pacific Time Zone stretches across the West Coast and is 3 hours behind.