The Earth's atmosphere surrounds the planet. It is a thin, clear, yet heavy ocean of air above our heads. It weighs down about 3 kg/cm². The atmosphere also protects the Earth. Comets, meteorites, dust, and other things burn up due to friction in the mesosphere. The atmosphere also keeps the Earth warm, acting like a blanket, due to the greenhouse effect.
When the Earth was forming it lost a lot of heat and volcanic gasses. Some of these gasses were kept close to the Earth by gravity. The composition of the atmosphere is as follows:
Water Vapor: 0-4%
Other Traces: ?%
Carbon Dioxide: 0.03%
The above are give/take figures. The atmospheric composition is not exact. The sky will not fall if there is 79% nitrogen instead of 78%.
Earth's atmosphere is made of many layers. They are as follows:
closest layer to Earth
contains weather/clouds/smog/75% of all of Earth's gasses/dust/ice/water
little temperature variation through stratosphere
converts oxygen into ozone
Ozone protects us from UV rays
melts meteors/space dust
ionosphere reflects radio waves at night/absorbs them during the day
H and He molecules escape from the exosphere into outer space
exosphere is The Edge
there is no fine distinction between the end of the exosphere and the start of space
Well, that's pretty much it for the atmosphere.