The air we breathe is actually a combination of several gases.  Within the first 80 km (Battan 11) or so in altitude, the composition is quite constant.  This portion is known as the homosphere.  (Higher in elevation, the concentrations of each gas vary considerably.  The table below shows the composition of the atmosphere (by volume) within the first 80 km.

The most significant constituent, Nitrogen, is held at a constant level through the nitrogen cycle.  Biological processes remove nitrogen from the atmosphere while decaying matter releases nitrogen.  Volcanic eruptions also release nitrogen into the atmosphere.  (Battan)  Since nitrogen is chemically stable, it doesn't need to interact with other gases.  Meaning that the nitrogen released billions of years ago is still around.  That is the reason that nitrogen makes up most of the atmosphere.  (Ackerman and Knox)

The second most abundant gas, Oxygen, is necessary for our survival.  Its consumption by animals, decaying matter, and other chemical reactions (Combustion, Oxidation, etc.) is balanced by its production by photosynthesis.  In the photosynthesis reaction, plants absorb carbon dioxide and release oxygen.  Oxygen is a result of one celled organisms.   Three billion years ago their byproduct of photosynthesis was oxygen today's oxygen is the result of that build up.  

Despite its small contribution to the total volume of air, the amount of carbon dioxide in the atmosphere is critical.  Carbon dioxide is a good absorber of infrared radiation emitted from the earth's surface.  Its rise in concentration has been blamed for the global warming phenomenon.  (Bear in mind however, that there are other "Greenhouse Gases").    Animals and the burning of fossil fuels emit carbon dioxide.   Carbon dioxide has increased since the Industrial Era meaning it's entering the atmosphere faster then it is being removed.   

The above chart also shows that there are a few trace constituents in the atmosphere.  These trace gases include Neon, Helium, Krypton, Hydrogen, Xenon, Methane, Nitrous Oxide, Ozone, and Chlorofluorocarbons (CFCs).

One extremely important gas in the atmosphere does not appear above.  Water vapor is highly variable in the atmosphere, ranging from nearly 0% to as much as 4%.  This large range can easily felt by our bodies.  (For instance, compare the extremely dry air on a cold, crisp winter's day to the moist, humid, and hazy air on a July afternoon).  The amount of water vapor in the air at any given time has a tremendous effect on the weather (such as aiding the development of a thunderstorm!)

And of course, the air contains thousands (per cubic centimeter) of small suspended particles and pollutants (Zabransky) called aerosols.  These can be anything from sea salt to smoke particles.  Some of these aerosols serve as Cloud Condensation Nuclei (CCN) and encourage the development of clouds and haze.

An important point should now be made.  In this web page, we will be dividing the atmosphere in several different ways.  For example, the atmosphere can be divided into two regions based on chemical composition.  Please note that the percentages shown only hold true for the first 80 or so kilometers above the earth's surface.  This relatively well-mixed layer is known as the homosphere.  Above this region, the heavier air molecules tend to settle out of the layer, leaving behind lighter gases such as hydrogen and helium.  The region from the top of the homosphere to space is known as the heterosphere.  This is one way that we can define different layers in the atmosphere.  The atmosphere can also be divided according to temperature and electrical properties.  You will see how this is done later in the tutorial.