A greenhouse is a structure, usually made of glass, in which temperature and humidity can be controlled for the cultivation or protection of plants. A greenhouse is designed to trap heat from the sun's rays inside and acts to keep the plants inside warm, even when it is cold outside. Although the Earth does not have a layer of clear material over it, certain molecules in our atmosphere absorb the Earth's heat, basically trapping some of that energy. This is called the greenhouse effect, and the molecules that trap the heat are called greenhouse gases.
Even though greenhouse gases don't make a hard surface like the glass of a greenhouse, but because they have a similar effect in keeping our planet warm, the term "Greenhouse Effect" is a good description. The greenhouse effect keeps the temperatures on our planet mild and suitable for living things.
Greenhouse gases (GHG) include carbon dioxide, water vapor, methane, ozone, nitrous oxide and fluorinated gases. These molecules in our atmosphere are called greenhouse gases because they absorb heat. There may not be much of some of these gases in our atmosphere, but they can have a big impact. These molecules eventually release the heat energy and it is often absorbed by another greenhouse gas molecule.
More technically: Greenhouse gases effectively absorb thermal infrared radiation, emitted by the Earth's surface, by the atmosphere itself due to the same gases, and by clouds. Atmospheric radiation is emitted to all sides, including downward to the Earth's surface. Thus, greenhouse gases trap heat within the surface-troposphere system. This is called the greenhouse effect. (Definition courtesy of IPCC AR4.)