Earth has seasons because its axis is tilted at about 23.5 degrees as it orbits the sun, causing different parts of Earth to receive more or less direct sunlight at different times of year. This tilt, not Earth's distance from the sun, is what creates summer, fall, winter, and spring.
How Earth's Tilt Creates Seasons
Earth spins on an invisible line called an axis that runs from the North Pole to the South Pole. This axis is tilted, not straight up and down. As Earth travels around the sun in its orbit, different parts of Earth tilt toward or away from the sun. When your hemisphere tilts toward the sun, you get summer with longer days and more direct sunlight. When your hemisphere tilts away from the sun, you get winter with shorter days and less direct sunlight.
Why Distance From the Sun Doesn't Cause Seasons
Many people think Earth has seasons because it moves closer to or farther from the sun, but this is not correct. Earth's orbit is almost circular, so the distance barely changes. In fact, Earth is closest to the sun in early January during Northern Hemisphere winter. The key factor is the angle at which sunlight hits Earth, which depends on the tilt of Earth's axis.
Opposite Seasons in Different Hemispheres
The Northern Hemisphere and Southern Hemisphere have opposite seasons at the same time. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. This happens because when the North Pole tilts toward the sun, the South Pole tilts away from it. The equator, which runs between the two hemispheres, experiences less extreme seasonal changes.
The Four Seasons
Spring begins when a hemisphere starts tilting toward the sun and has equal day and night length. Summer is when the hemisphere tilts most directly toward the sun, with the longest day of the year. Fall begins when the hemisphere starts tilting away from the sun, again with equal day and night. Winter occurs when the hemisphere tilts most away from the sun, with the shortest day of the year. 
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