A season is a division of the year marked by changes in weather, ecology, and the amount of daylight. On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant.

Various cultures define the number and nature of seasons based on regional variations.

In temperate and sub-polar regions, four seasons based on the Gregorian calendar are generally recognized: spring, summer, autumn or fall, and winter.

More Info: en.wikipedia.org