The original cloche was a clear bell-shaped jar made to protect tender plants, not just from the cold but also from insects, birds and strong winds. Very simple. Very effective. In an emergency, a cloche can be improvised with an empty jam jar - but it can only be left in place for a short time because of the lack of ventilation.
The limitation of a bell cloche is its capacity. A cloche is cabable of covering one plant or a collection of small seedlings. For gardening on a larger scale, or indeed agriculture, the cloche becomes somewhat impractical. As result, the continuous cloche was invented. A continuous cloche is capable of coving whole rows of plants. The continuous cloche, or row cover, is a tunnel-like structure normally made of plastic and supported by wire or plastic hoops.
Continuous cloches are now preferred. They are available glass, plastic, fleece and mesh coverings. Glass is more traditional, more aesthetically pleasing and resistent to solar degradation. Plastic is cheaper and less breakable. Plastic is also lighter - good for transporting around but not good for staying in position. This problem is resolved if you place soil in the wide lower lip to hold the cloche in place.
In spring, delicate young seedlings can be planted out weeks earlier than would otherwise be feasible under the protection of a cloche. This is particularly desirable in the case of vegetable crops such as tomotoes and peppers. Using a cloche will also extend the season for crops in the autumn/fall by up to 3 weeks.