History
Historically cold frames were built to be used in addition to a heated greenhouse. The name, itself, exemplifies the distinction between the warm greenhouse and the unheated cold frame. They were frequently built as part of the greenhouse's foundation brickwork along the southern wall (in northern latitudes). This allowed seeds to be germinated in the greenhouse and then easily moved to the attached cold frame to be hardened-off before final planting outside.

Characteristics
Cold frames are found in home gardens and in vegetable farming. They create microclimates that provide several degrees of air and soil temperature insulation, and shelter from wind. In cold-winter regions, these characteristics allow plants to be started earlier in the spring, and to survive longer into the fall and winter. They are most often used for growing seedlings that are later transplanted into open ground, and can also be a permanent home to cold-hardy vegetables grown for autumn and winter harvest.

Features and Benefits
One of the advantages of using a cold frame is that you can raise a huge variety of crops from seed through to maturity within the frame - so it's possible to have vegetable crops ahead of their normal season when they are extremely expensive to buy. Some crops suitable for growing in a cold frame include lettuces, parsley, salad onions, spinach, radishes and turnips etc. One vegetable crop can occupy the whole of a cold frame or you can grow a combination of crops so that they mature in rotation - so you'll get a wide range of different vegetables throughout the year from one cold frame.