What is the Earth's radiation budget?
Like any budget, the Earth's radiation budget represents a balance between incoming and outgoing stuff, in this case that stuff is radiation.
How does it work?
The incoming radiation is from the sun, and is generally called shortwave radiation because it is typically wavelengths on shorter end of the spectrum. The outgoing radiation comes from the Earth's surface, and is generally called longwave radiation for reasons identical to those of the shortwave radiation. Although the temperature changes from day to day at specific locations can be quite large, the average temperature of the whole planet is relatively constant. Therefore the incoming and outgoing radiation must be in balance. If there was more outgoing radiation than incoming radiation the planet would cool down (lose more energy than it gained), and conversely if there was more incoming radiation than outgoing the planet would warm up.
The role of the atmosphere
Without the atmosphere, the Earth would be on average roughly 50C colder than it currently is. This is because the atmosphere acts as a greenhouse, letting most of the shortwave solar radiation through to the surface but slowing the outgoing longwave radiation long enough to keep the planet's average temperature up. There are a number of chemicals in the atmosphere that make this possible, of which water vapor and carbon dioxide are the most famous. The more water vapor and carbon dioxide in the atmosphere, the less longwave radiation is able to make it back into space and the temperature of the planet increases. Unchecked carbon dioxide emissions can cause a runaway greenhouse effect in which the Earth's temperature will continue to rise uncontrollably. Most scientists agree that we are beginning to see temperatures around the globe increase as a result of higher levels of greenhouse chemicals in the atmosphere.
Artwork by Maija Swanson and Nick Olejniczak