The sun heats the earth through radiation. Since there is no medium (like the gas in our atmosphere) in space, radiation is the primary way that heat travels in space.
Heat coming out of Radiation from the Sun
When the heat reaches the earth it warms the molecules of the atmosphere, and they warm other molecules and so on.
Heat entering the atmosphere, warming all molecules.
Equator: Top is Northern Hemisphere. Bottom is Southern.
As you can see, if the Northern Hemisphere (top of the equator) is towards the Sun, it's summer on top. Countries like America, Russia, Europe and more is summer. This is because the top receives direct sunlight.
However, same goes to the Southern Hemisphere. If it faces the Sun, it's summer. So in other words, if either of the sides faces the Sun, it's summer.
But, any countries that go by the line (equator), will not have any seasons.
So, whatever side of the Earth faces away from the Sun, it's winter. This is because it doesn't receive direct sunlight.
So why do seasons exist? This is because the Earth tilts on its axis; showing each country receive direct sunlight and the other side not.