Although oxygen is probably the most familiar gas in the atmosphere to us, it is not the most dominant. 78% of Earth's atmosphere is composed of nitrogen. 21% is oxyten, 0.9% is argon, and a mere 0.03% is carbon dioxide. Water vapor makes up an addition 0.1-->0.3%.
We can divide the atmosphere into 4 layers based on its temperature structure. The lowest layer is the troposphere. Within this layer, temperature increases as you rise in elevation. Weather occurs in this layer and clouds predominately occupy the boundary between this layer and the next one. Above the troposphere we find the stratosphere, which gets warmer with elevation, due to the fact that this layer absorbs UV radiation from the Sun. The ozone layer is at the top of the stratosphere. Together, the troposphere and the stratosphere contain about 99% of the gas in the atmosphere (by mass). Above the stratosphere is the mesosphere, where temperature again drops with altitude. Finally, the outermost layer is the ionosphere, or the thermosphere/exosphere, containing gas that has been ionized by solar radiation. Density gradually drops off the farther you travel from the Earth's surface.
Heat energy is transfered throughout the troposphere via convection. Hot air is less dense than cooler air, this is why hot air balloons float. The ground is heated by solar radiation, which in turn heats the air near the ground. This air rises and cools and then the cooled air once again sinks. This process creates air turbulences, which airplanes try to stay above.
If it were a perfect blackbody, the average temperature on earth would balance out at around 250K, about 20K below the freezing point for water. Brrrr... This equilibrium is determined by the point at which the energy of the blackbody radiation being emitted by Earth (due to its temperature) matches the energy it absorbs from the Sun. Earth is not this cold because of the "greenhouse effect." Optical light from the Sun passes easily through the atmosphere. Once it is re-radiated by Earth's surface as thermal radiation, however, it has been transformed into infrared radiation. Carbon dioxide and water vapor in the atmosphere block this radiation from leaving, raising the temperature on the Earth to levels hospitalable to us. This part of the greenhouse effect is good. Unfortunately, it is highly sensitive to the amount of CO2 and H2O in the atmosphere. If too much energy is trapped in the atmosphere, things could get too hot.
Earth's atmosphere scatters some of the optical light passing through it. This scattering effect is more pronounced for shorter wavelength (bluer) light. Thus, some of the Sun's blue light is scattered throughout the atmosphere, coloring the sky. In turn, the Sun appears somewhat redder than its true color. At sunset, this light must travel through even more atmosphere, resulting in an even redder Sun.
Tides are caused by the difference in the strength of the Moon's pull on the side of the Earth facing the Moon, versus the Earth's center, versus the opposite side of the Moon. This results in a pair of tidal bulges on the Earth, one pointing towards the Moon and the other pointing away. The size of this bulge is about 1m for water and 1cm for land. As the Earth rotates, different places on the Earth experience the tide.
The Sun also raises tides on the Earth, though much smaller ones than those raised by the Moon. At full/new Moon when the Sun, the Earth, and the Moon all line up, these tides reinforce each other and we have the more-pronounced spring tides. At first/third quarter moon, there is some cancellation and we have weaker "neap" tides.
Because the Earth is rotating faster than the Moon moves around the Earth, the tidal bulge gets pulled along by the rotation of the Earth. This creates friction, gradually slowing the Earth's rotation. Over a few billion years, this will eventually cause the same side of the Earth to always face the Moon. This process already occurred to the Moon, which is why its same face always is aimed towards the Earth.
The Earth has its own magnetic field called the "magnetosphere." This field acts as a barrier to various high-energy particles in the solar wind that would be very harmful to life. Charged particles get trapped in the field and sometimes rain down over the poles as aurora.
This magnetic field is generated by the rapid rotation of its liquid outer core. Every 200,000 or so years, the polarity of this field reverses. It has been 800,000 years since the last reversal and some scientists claim to see evidence that the next reversal may have begun. The reversal itself may take a few thousand years to complete itself.
