Some notes on the Greenhouse Effect
The Earth receives energy from the Sun. During the hours when some area of the surface is illuminated, a large fraction of the energy of electromagnetic radiation that falls on it is absorbed, and converted to heat. However, due to blackbody radiation, at the same time the earth's surface is radiating away energy (mostly in the form of infrared light); during the hours of illumination, it absorbs more than it radiates away. At night, the Earth's surface continues to radiate as a blackbody radiator, but there is nothing to replace the energy that is lost, so the surface cools during the night time.
If the Earth had no atmosphere, that would be the end of the story. But the atmosphere alters both the amount of sunlight that falls on the surface (reducing it through absorption and scattering), and the amount of blackbody radiation that, having been emitted by the Earth's surface, makes it out of the atmosphere, and on out into space. The effect on the outgoing radiation is larger than the effect on the incoming radiation, and so the Earth's surface is warmer than it would be (by about 19 K, on average) if there were no atmosphere.
Though a number of processes contribute to the effectiveness of the atmosphere at blocking the radiation loss from the Earth's surface, in essence they all result in the reflection (usually by absorption and re-radiation) of the energy back from the atmosphere to the surface, where it is again absorbed. The processes responsible for this reflection are very much affected by the concentrations of certain kinds of molecules in the atmosphere. Those chemical compounds (molecules) that are particularly effective are often called greenhouse gasses. For example, oxygen and nitrogen themselves are not very effective, and so are not considered to be greenhouse gasses. However, water vapor, carbon dioxide, and some of the CFCs are greenhouse gasses. When the concentrations of these molecules in the atmosphere increase, there will be a net warming of the Earth's surface, and of the atmosphere immediately above it.
An interesting point about the term "greenhouse effect" is that the atmospheric phenomenon that goes by this name has very little to do with how a greenhouse works. The transparent walls of a greenhouse do not trap outgoing radiation from the inside of the greenhouse. Rather, the ceiling of the greenhouse prevents the inside air, as it is being heated, from rising, and being replaced by cooler air from around the outside of the structure. The same thing happens to any enclosed space (automobiles, and even opaque-walled sheds, for example) that is being heated.