The concept of hardiness is of most significance in regions with cold winters. The lowest temperature that a plant can survive regulates where it can be grown successfully. Tender plants cannot survive a frost and therefore can only be grown in regions or seasons that are frost free. Most bedding plants (Pelargonium, Impatiens etc.) fall into this category.
Hardy plants can survive a frost. The extent to which they can tolerate cold weather depends upon a mix of species physiology, duration and magnitude of sub-zero temperatures, soil type/drainage and numerous other factors.
Hardiness zones have been defined for Europe and the USA based on isotherm maps joining areas of equal low winter temperatures. These are useful as guidelines but local conditions can vary a great deal, making the maps of limited practical use.
US zone map follow this link - http://www.usna.usda.gov/Hardzone/ushzmap.html