Greenhouses are what most people think of when “Indoor Growing” is brought up. That is because traditionally they have been the center of indoor growing. They provide the benefits of better control of all the environmental factors such as temperature, humidity, CO2 Levels, and so much more like all indoor growing methods. But many places / crops will not require Growing Lights in a Greenhouse unlike the other types of Indoor Growing Environments. This is clearly due to the Greenhouses are usually covered in a transparent material such as glass, plastic, plexiglass, and such. These materials allow natural sun through. 

Greenhouses come in many different forms and size factors from massive commercial greenhouse complexes to tiny “cold frames” that you can’t even walk in. They have many different structural designs from “hoop houses” to more traditional framed greenhouses. They can also range from very economical plain metal frames covered in plastic to very expensive / beautiful iron frames with glass. The types, designs, and purposes are almost endless with Greenhouses.