The Earth's seasons are the result of the tilt of the Earth's axis. This tilting is what gives us the four seasons of the year - spring, summer, autumn (fall) and winter. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year.
Summer is warmer than winter (in each hemisphere) because the Sun's rays hit the Earth at a more direct angle during summer than during winter and also because the days are much longer than the nights during the summer. During the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short.
The spring equinox brings the beginning of warmer longer days. Leaves bud, and flowers grow. :) In autumn, the plants are preparing for their winter dormancy. They're responding to the shorter days and cooler temps. Animals have developed cycles to respond to the seasas well.
