A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of transparent material, such as glass, in which plants requiring regulated climatic conditions are grown. These structures range in size from small sheds to industrial-sized buildings. A miniature greenhouse is known as a cold frame. The interior of a greenhouse exposed to sunlight becomes significantly warmer than the external ambient temperature, protecting its contents in cold weather.
Sea of plastic Almeria Green Houses in Spain (Source: Google Maps)
A polytunnel (also known as a polyhouse, hoop greenhouse or hoop house, grow tunnel or high tunnel) is a tunnel typically made from steel and covered in polythene, usually semi-circular, square or elongated in shape. The interior heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. Temperature, humidity and ventilation can be controlled by equipment fixed in the polytunnel or by manual opening and closing of vents. Polytunnels are mainly used in temperate regions in similar ways to glass greenhouses and row covers. Besides the passive solar heating that every polytunnel provides, every variation of auxiliary heating (from hothouse heating through minimal heating to unheated houses) is represented in current practice. The nesting of row covers and low tunnels inside high tunnels is also common.
Multi-Span Polytunnel (Source Wikipedia)