Definition (Source: Wikipedia)
***A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. These structures range in size from small sheds to very large buildings.
Mainly you can divide in greenhouses in glass greenhouses and in plastic greenhouses. The glass greenhouses are high tech production facilities for vegetables or flowers. They are built mainly by Dutch greenhouse builders. These Dutch greenhouses builders are the leading companies in this field. The glass greenhouses are filled with equipment like screening installations (to keep out direct solar radiation and / or energy savings), heating (for maintaining the desired temperature inside the greenhouse for the crop e.g. 18 degrees Celsius, also in wintertime), cooling, lighting (to get more solar radiation for growing the crops) and these installations are all automaticly controlled by a computer.