Greenhouse gases are a natural and important part of the Earth's atmosphere. Without them, the global climate would be an estimated 65F (20C) cooler. It works like this: the Sun emits enormous amounts of radiation, in a wide range of wavelengths. As the radiation approaches the Earth, it encounters the Earth's atmosphere, and is largely reflected back into space or absorbed. Only about 35% of the radiation coming from the Sun actually makes it to the surface of the planet. That 35% is absorbed by the Earth and transformed from light energy into heat energy, which is re-emitted back into the atmosphere. The heat would escape into space, leaving the Earth a cold, lifeless rock, except that certain gases in the atmosphere absorb and re-release it. By continually absorbing and releasing heat in an endless cycle, the gases trap heat in the atmosphere, and the atmosphere is warmed. The warm blanket of air surrounding the Earth helps to keep it warm, too, allowing life to flourish. So present concerns about greenhouse gases aren't that they exist in the atmosphere, since we need them, but that, as a result of man's activities, concentrations of the gases are increasing to the point that we are beginning to experience negative effects from unwanted, "enhanced," global warming.
The most abundant greenhouse gas is water vapor. But as you know from your experiences with changing humidity, the amount of atmospheric water vapor varies from place to place and over time, making it extremely difficult to control. Climate scientists and environmentalists seldom mention water vapor, and focus on the other, more manageable, greenhouse gases.
After water vapor, the most abundant greenhouse gas is carbon dioxide. Prior to the Industrial Revolution, when man began burning enormous amounts of fossil fuels, carbon dioxide levels in the atmosphere are estimated to have been about 280 parts per million by volume (280 ppmv). That means if the atmosphere were 1,000,000 square meters, then 280 of them would be occupied by CO2. Atmospheric carbon dioxide has increased steadily since the late 1800s to the current level of about 360 ppmv. Carbon dioxide is not really a very powerful greenhouse gas, but because it is relatively abundant, its effect is most significant. But CO2 absorbs radiation in only a narrow bandwidth, and some scientists have argued that the bandwidth is basically "saturated," and that further increases in the concentration of that particular greenhouse gas really won't affect global climate appreciably.
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