Most seed and plant catalogues include a USDA Hardiness Zone Map based on national climate data. The map (published in 1990) puts my yard firmly in zone 7; the finely detailed versions of the map, like this one, show my yard as being on the edge between zones 7a and 7b.
Checking the plant hardiness zone map is important in determining what plants to buy and when to plant them, to avoid potentially costly mistakes. No one will want to lose either the money or the effort that goes into planting and caring for plants that are not suited to a particular yard or that have been planted out too soon!
Interestingly, the map in most of my catalogues is based on older data. In 2003, the American Horticultural Society (AHS) issued a new map (pdf file and really interesting article) based on a more recent set of climate data—from 1986-2002— for the U. S. The newer map shows a big swath of zone 8 right up into north Georgia, with a little patch of zone 7 just NW of metro Atlanta. The map does not include the half zones (a and b divisions).
The Arbor Day Society also created a new hardiness zone map (in 2006), using the same data source (but with more recent years) as the USDA did for the 1990 map. This map looks a lot like the AHS map, and also puts Atlanta in zone 8. However, the Arbor Day Society has provided a great little tool to let people look up their hardiness zone as shown on the new map. The results show that my zip code is in both zones 7 and 8 (apparently, my yard is still “on the edge”) and includes a disclaimer about the effects of microclimate as an influence as to which way my yard actually swings.
Ever since seeing the new map, I’ve been careful that plants I order are successful down to zone 8, just in case. Some plants have a minimum "chill" time, or days of freezing weather requirement, below which they don't set fruit. This requirement might not be met in zone 8 for a "hardy for zones 5-7," for example, plant.
For gardeners who want to be extra-careful in choosing plants for the yard, it is also useful to look at the AHS Heat Zone Map (another pdf file) since some plants can tolerate cold but not excessive days of heat. Anyone who reads garden blogs whose writers live in the Pacific Northwest will understand the difference the heat makes. Many of those gardeners can grow salad right through the summer. Even though our hardiness zones may be identical, here in Georgia the summer heat sends all the cool weather veggies into bitterness and bolting.