There are several different climate zone map systems devised for the United States. The first and most commonly known climate system is the USDA Hardiness Zone Map. The first map was issued in 1960. It was revised in 1965. The next time a new map came out was in 1990. This map was based on more reporting stations and it and it divided the temperature zones into five-degree a/b zones for greater accuracy. This map identified some areas as colder than the previous map due to some extremely low temperatures in the intervening years. The latest map came out in 2012. This map takes into account the warmer temperatures of the previous 30 years. Many areas moved up by a half zone and urban heat islands are taken into account. Depending on where your weather reporting station is located you may be in a different subzone or even a different zone. The difference between an inner city garden near downtown and the weather station located on a open and windy airport 20 miles out in the country can be quite great.
The zones are color coded to be more visible. One interesting thing to notice is that the zones are not even east to west across the country. There are a number of reasons for this. Higher Elevations tend to have cooler temperatures and also areas near oceans and large bodies of water have more moderate temperatures. An extreme example is some areas near Seattle Washington have a Zone 9 A Climate. This is the same zone as Jacksonville Florida. Remember though that the USDA Zones record the minimum temperature. Other factors such as moisture, hours of sunlight and summer heat are important as well.
The USDA has a great website http://planthardiness.ars.usda.gov/PHZMWeb/# which explains the USDA Hardiness Zone Map and how to use it. You can search your zone by zip code. It’s a fun feature to play with. Where I live in Toledo Ohio the city is divided into zones 6A and 6B with areas closer to downtown and Lake Erie 6B and more western areas 6A.