In a sense, atmosphere functions like a blanket, keeping Earth's heat from escaping into space. It has also been compared to a greenhouse: like glass it lets short wave insolation inside, but keeps most of long wave ground radiation from going out. Your car is an example of a 'real' greenhouse. Short and long wave radiation enter and are absorbed. Some short wave radiation is reflected (especially if the interior of your car has a high albedo), but the longwave radiation that is released from your heated up interior is trapped by the glass. That is why when your car has been sitting in a parking lot all day it is much hotter than the outside temperature. A true greenhouse lacks circulation. When you get into your hot car, it is likely that you immediately roll down the windows to cool the car off (or let all that trapped longwave radiation out!).
The term greenhouse effect is often used to describe this key property of the atmosphere. However, it is important to note that our atmosphere is not a true greenhouse. Water vapor and CO2 are known as "greenhouse gases" (there are other "greenhouse gases" as well) because they act like the glass in your car, trapping longwave radiation, and encouraging the formation of thin high altitude clouds. You might want to research and study the greenhouse effect carefully. Why is the ‘greenhouse’ analogy not fully applicable to our atmosphere?
K. Allison Lenkeit-Meezan (Foothill College)