A cold frame is simply a raised bed garden that serves a unique purpose. Cold frames are beds that have a cover of some type to protect tender young seedlings from the ravages of the spring frosts. Many times they are simply a cover that goes over the bed which is intended to be the final destination of the plants, and just provides early protection and allows the plants to “harden off” or acclimate to the variations in sunlight, moisture, and temperature.
Other cold frames are smaller boxes that are intended primarily for hardening off seedlings, and they are taken from the cold frame and planted in the garden after the danger of frost has past.
Either way, these are a great way to extend the growing season. This is true not only for colder climates, but can be used in some warmer climates like the Southwest and Texas where the summers can be too hot for some plants like tomatoes, and getting an early start allows the plants to mature before the heat kicks in.