Why do the seasons change?
The short answer
The main reason for the seasons is Earth's axis, which is tilted about 23 degrees as Earth travels around the Sun. The tilt changes which hemisphere receives the most direct sunlight.
The difference between summer and winter
When the Northern Hemisphere tilts toward the Sun, the Sun travels higher in the sky and daylight lasts longer. More direct sunlight reaches the ground, so the Northern Hemisphere tends toward summer.
When it tilts away, sunlight arrives at a lower angle and the days are shorter. The ground receives less energy, so it tends toward winter. The seasons are reversed in the Southern Hemisphere.
Is distance from the Sun the cause?
Earth's distance from the Sun changes a little during its orbit, but that is not the main cause of the seasons. The angle of sunlight and the length of daylight caused by Earth's tilt matter much more.