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The greenhouse effect is the warming of the temperature of the air, caused by the temporary trapping of radiant heat. The word itself is a little misleading, because it sounds as though it only happens in a greenhouse. However, the greenhouse effect is in our whole atmosphere. Radiant energy from the sun passes down through the atmosphere is absorbed at the surface of the Earth. Some infrared energy is re-emitted to the atmosphere. Carbon dioxide molecules then absorb and re-emit infrared energy. Infrared energy escapes to space more slowly when the level of carbon dioxide and other 'greenhouse' gases increases. 
As in a real greenhouse, the greenhouse effect works in a cycle. The radiant energy from the sun goes through the glass on the roof and walls of a greenhouse. The sunlight then touches things in the greenhouse, like plants and tables, etc. These warm up and emit heat (infrared energy), which escapes back through the glass more slowly than the energy coming in, causing the heat to build up inside. (When it gets warm enough, the heat will be able to leave as fast as the energy coming in, and equilibrium will be reached ... the temperature will stop rising). 
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