Since the Earth is a sphere, different parts of the surface will receive different amounts of sunlight at any given point in time. If the Earth was flat then the entire surface facing the sun would be evenly heated. But since the surface is curved, the area near the equator is the closest to the Sun while the two poles are furthest from the Sun. This is why the equator is a tropical area and the poles are frigid. Furthermore, the Earth's axis sits at a tilt from the perpendicular. This tilt also rotates (this is called precession). The ultimate result of all of this is that different hemispheres of the Earth are closer to the Sun at different times of the year, thus giving rise to the seasons. This is also why the seasons are opposite for the two hemispheres (when its winter in the north its summer in the south).