The earth is slightly tilted on its axis. As the sun shines on the earth, it shines more directly on the northern hemisphere in June, and more directly on the southern hemisphere in December. That's why the seasons are different in each hemisphere. In the spring and fall, the sun shines fairly straight on the equator, giving both hemispheres equal warming.
Experiment:
Global Lighting
Take a globe and a light bulb or flashlight. Imagine the globe is the earth and the light is the sun. In the summer, what would be the position of the earth? What is the earth's position in each of the other seasons?
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