Greenhouses are normally made from glass or polycarbonate. What they do in simple terms is convert light into heat. The sun's rays are converted by the greenhouse in the form of solar energy and converted into heat, also known as thermal energy. The best way to think about it is to imagine how a car with its windows closed heats up when parked in the sun.
As the greenhouse heats up, the objects in the greenhouse absorb this heat and store it. The best items to place within the greenhouse are ones with good thermal mass such as soil, bricks and water. Once the sun has gone down, these items then release the stored heat back into the greenhouse. This is also particularly beneficial to keep the greenhouse warm on days when the sun isn't shining.
Greenhouses work best when they are positioned in an area where they can absorb as much thermal energy as possible. They should not be built in areas that receive any kind of shade throughout the day or in front of a building that will block sunlight. You can optimize the efficiency of a greenhouse by making sure it is well ventilated, that the soil is well watered and that there are plenty of insects inside to allow for the cross pollination of the plants. In colder climates, heating devices may be required to heat the greenhouse during the winter months.
Greenhouses also work by keeping plants protected from the external environment and allowing them to grow to their full potential. They stop plants from being damaged by too much wind or rain and remove the problems of being eaten or trampled by animals.