Source: National Weather Service forecast Office: http://www.crh.noaa.gov/fsd/astro/season.htm
The Earth has an elliptical orbit around the Sun. The Earth is at its closest point distance wise to the Sun in January and furthest in July. Near June 21st, the summer solstice, the Earth is tilted such that the Sun is positioned directly over the Tropic of Cancer at 23.5 degrees north latitude. This situates the northern hemisphere in a more direct path of the Sun's energy. What this means is less sunlight gets scattered before reaching the ground because it has less distance to travel through the atmosphere. In addition, the high sun angle produces long days. The opposite is true in the southern hemisphere, where the low sun angle produces short days. Furthermore, a large amount of the Sun's energy is scattered before reaching the ground because the energy has to travel through more of the atmosphere. Therefore near June 21st, the southern hemisphere is having its winter solstice because it "leans" away from the Sun.
Advancing 90 days, the Earth is at the autumnal equinox on or about September 21st. As the Earth revolves around the Sun, it gets positioned such that the Sun is directly over the equator. Basically, the Sun's energy is in balance between the northern and southern hemispheres. The same holds true on the spring equinox near March 21st, as the Sun is once again directly over the equator.
Lastly, on the winter solstice near December 21st, the Sun is positioned directly over the Tropic of Capricorn at 23.5 degrees south latitude. The southern hemisphere is therefore receiving the direct sunlight, with little scattering of the Sun's rays and a high sun angle producing long days. The northern hemisphere is tipped away from the Sun, producing short days and a low sun angle.
