The words "frost" and "freeze" are being tossed around a lot in the weather forecasts lately. The National Weather Service considers 32 degrees to be a frost and 28 degrees to be a freeze.
However, this is rather ambiguous. After all, 32 degrees is, in fact, freezing, and frost does not necessarily form at 32 degrees unless the air near the ground becomes saturated with moisture. Other organizations, including the American Meteorological Society, define "frost" as the icy crystals that sometimes cover the ground in freezing weather, whereas "freeze" is when the air temperature goes below freezing in conditions too dry for frost to form.
From a farmer's or gardener's standpoint, a killing freeze is when a plant's cell structure is damaged by the process of water freezing in the plant, which is really determined by the biology of a particular plant and whether or not the plant is exposed to freezing temperatures long enough for damage to occur.