Today is Summer Solstice, which marks the beginning of Summer for those in the southern hemisphere. On this day the sun follows its highest path in the sky. Starting tomorrow the Sun will trace a lower and lower daily path until it reaches the Winter Solstice in December. Of course most people live in the northern hemisphere, and for them today marks the Winter Solstice. Contrary to popular belief, the seasons are not caused by our distance from the Sun. The Earth is actually closest to the Sun (at perihelion) on January 3, and farthest (at aphelion) on July 4. Although the changing distance from the Sun does have an effect on the Earth's temperature, it is tiny when compared to the variation of the Sun's path across the sky. It is our orientation relative to the sun, not our distance, which is the cause of our seasons.
The axis of the Earth is tilted about 23 degrees relative to its orbital plane. As the Earth orbits the Sun, its relative orientation changes, as seen in the figure below. At the Winter Solstice, the pole of your hemisphere is tilted slightly away from the Sun. As a result we receive less heat and light, which is why it is colder. As a result the seasons in the southern hemisphere are opposite to the seasons in the northern hemisphere, which is why Santa celebrates at the beach in Australia.
The Winter Solstice is one of four dates marking the cycle of the Sun's daily path during the year. The Summer Solstice marks the longest day of the year, when the Sun follows its highest path in the sky, typically on June 21. Halfway between these two extremes are the Spring and Fall equinoxes, typically on March 21 and September 21. This is, of course, for the northern hemisphere. For the southern hemisphere Summer and Winter are reversed.
Since the dawn of history, human civilizations were aware that this variation of the Sun's path could be used to predict the cycle of the seasons. When the Sun moved lower in the sky it meant Winter was coming. When the Sun began to follow a higher path in the sky, warmer days would follow. It's reliability was so strong that early civilizations made careful measurements of the cycle, and even held celebrations to mark them. Image:
Swinburne University of Technology (http://goo.gl/iExNZ