What is a cold frame? A cold frame is simply a translucent or clear box without a bottom that is used to protect tender plants from the cold weather of late fall and early spring.
Most people who go about building a cold frame will use a wooden base and then a glass or plastic window over the top. (Old recycled windows make for a wonderful cold frame top.) The cold frame uses solar energy to trap heat inside, warming the ground and plants, to allow you to extend your growing season past the first frost. It can also be used to harden off seedlings that were started indoors—ensuring their survival through the season—or for seed starting directly in the ground under the frame.
A cold frame can be made simply, or it can be relatively complex. Remember all it has to do is protect plants and hold in heat. A simple cold frame can be made from recycled milk jugs. Just cut off the bottom and bury the edges slightly in the dirt over a plant. On a sunny day, the milk jug will trap heat in the same way the glass windows will, and you can open the caps to allow for ventilation.
If you want to get more complicated you can make a wooden frame and use windows that open and close and create hot beds. A hot bed is a cold frame that has electric heating inside. If you prefer not to use electricity you can also dig down into the ground about 15 inches and fill the bed with manure around your plants. As the manure breaks down it will create heat to warm your plants at the same time it feeds them.
Come spring you can sow seeds weeks earlier if you utilize a cold frame; combining that with the longer growing season into fall you can add a whole two months or more to your growing season.