Because it is a small area here in the UK we have just one time zone, and everyone is on the same time.
The US, being so wide, spans a number of time zones (each one hour difference) and they are known as:
ET - Eastern Time (New York)
CT - Central Time (west of New York)
MT - Mountain Time - (Almost on west coast)
PT - Pacific Time (San Francisco)
So when you are in New York (ET) the time in San Fancsisco (PT) is 3 hours earlier.
More here:
http://www.timetemperature.com/tzus/time_zone. shtml