In short, we have seasons because the Earth's axis is tilted.
As the Earth spins on its axis giving us night and day, it also revolves around the Sun in an elliptical orbit (or elongated circle).
The trip around the sun takes 365.25 days to complete.
Since the Earth's axis is tilted at 23.5 degrees, the most intense sunlight will hit different parts of the Earth at different times during its revolution around the Sun.
The amount of direct sunlight you get determines which season you are experiencing.
Therefore, when the Earth's axis points towards the Sun, there is more direct sunlight for the northern hemisphere. As a result, they experience summer while the southern hemisphere has winter.
When the Earth's axis points away from the sun, winter occurs in the northern hemisphere and the southern hemisphere has summer.