The change of the seasons on Earth result from the changes in the way in which the Earth's axis is tilted in relation to the sun, and the position of the Earth on its orbit around the sun.
The Earth's axis is the imaginary line that runs through the planet, connecting the North and South Poles, with the Earth spinning on this slanted axis. As the Earth revolves around the sun, the 23.5 degree tilt of the axis results in the sun's rays more directly hitting either the northern or southern hemisphere. When the northern hemisphere is receiving more intense and direct sunlight, it is summer there and winter in the southern hemisphere. When the southern hemisphere receives the more direct sunlight, it is summer in that part of the world and winter in the northern hemisphere.