The Earth has both a tilt and a wobble and the two of those cause the seasons. The area on Earth that is closest to the sun get summer and the area that is furthest gets winter. As the Earth wobbles, the seasons change. You can find more information here: https://pantherfile.uwm.edu/kahl/www/CoVis/Seasons/

Seasons change because of the tilt of the Earths axis. While the earth moves around the sun it is also rotating on it's own axis. This movement causes different parts of the earth to lean away from the sun, which causes lower temperatures. You can find more information here: http://www.scienceu.com/observatory/arti...