Many people believe that Earth is closer to the Sun in the summer and because of that it is hotter. In the same way , we think Earth is farthest from the sun in the winter and that's why it is cold.
But actually this idea is not correct. In Northern Hemisphere, we are having winter when Earth is closest to the sun and summer when it's farthest away.
Image credit:
https://spaceplace.nasa.gov/seasons/en/
Earth's has seasons because 1) Earth's orbits around the Sun is not perfect circular but elliptical. 2) Earth's axis is not straight and it is tilted.
Earth's axis is an imaginary pole going right through the center of the Earth from top to bottom. Earth spins around this pole and making one complete turn each day. Thats Why we have day and night.
The Earth has a tilt of 23.5 degree relative to the ecliptic plane(the imaginary surface formed by Earth's circular path around the sun.)
Image Credit:
https://en.m.wikipedia.org/wiki/Effect_of_Sun_angle_on_climate
According to the figure when Sunlight shines on the Earth at a lower angle(Sun Closer to the horizon) the energy of the sunlight is spread over a larger area, and is therefore weaker than if the Sun is perpendicular and the energy is concentrated on a smaller area.
Image Credit:
https://en.m.wikipedia.org/wiki/Effect_of_Sun_angle_on_climate
According to figure, we know that the angle of sunlight striking the Earth in the Northern and Southern hemispheres when the Earth's northern axis is tilted away from the Sun, when it is winter in the North and summer in the south.
As Earth orbits the sun, its tilted axis always points in the same direction. So, throughout the year,
different parts of Earth get the sun's direct rays.
In the December, on the date of the winter solstice, the Earth's tilt away from the sun is maximimum and that's why minimum sunlight reaching the Northern Hemisphere. Because of that seasons of Southern Hemispheres is opposite and it is summer during that time.
As amount of lands surface in Northern Hemisphere is greater than Southern Hemisphere and since Land Heats faster to a higher temperature than the Ocean, and they cooled down faster too because specific heat of water is higher than land. Water need higher heat to raise its temperature compared to land. The Southern Hemisphere summer is not too much hot as compared to Northern Hemisphere Summer.
The sun is actually closest to the Earth during Northern Hemisphere winter (not summer). Thus, the amount of sunlight averaged over the whole Earth, is as much as 7% more intense in the winter than the summer. Despite this fact, the global-average surface temperature is warmer in Northern Hemisphere summer, due to the much greater expanse of land there, and since land heats to a higher temperature than the ocean does.
More Information:
Source:
https://spaceplace.nasa.gov/seasons/en/
https://en.m.wikipedia.org/wiki/Effect_of_Sun_angle_on_climate