Quick answer: The seasons occur because of the tilt of the Earth’s axis of rotation.
Each year the Earth travels through space orbiting around the Sun. As it does so we observe the passing of the seasons. Although the amount of variation in the seasons depends on where we live, the trends are the same.
During summer the length of the day exceeds 12 hours. So in summer, nighttime is shorter than daytime. During summer is when we experience the longest days and shortest nights compared to any other time of year. The Sun is higher in the sky at midday in summer than in winter. The combination of longer days and the high Sun explains the heat of summer.
By contrast, winter is characterized by shorter days and longer nights. Nighttime in winter exceeds 12 hours. With the Sun lower in the sky at midday, the sunlight striking the ground is less concentrated. This results in the significantly cooler temperatures of winter.
Another fact is that summer in the northern hemisphere coincides with winter in the southern hemisphere. Likewise, winter in the northern hemisphere coincides with summer in the southern hemisphere. In between summer and winter we have the seasons of spring and autumn during which the length of day and night are roughly equal over most of planet Earth.
All these effects are linked to the same cause, the tilt of Earth’s rotation axis. The rotation axis of the Earth is inclined by about 23½° away from the axis of Earth’s orbit around the Sun. Like a spinning top, the Earth’s rotational axis points in the same general direction in space. So as Earth orbits around the Sun, one hemisphere (northern or southern) may be tilted toward the direction of the Sun while the opposite hemisphere tilts away.
Summer in the northern hemisphere occurs when the north pole is the most tilted toward the Sun. At this same time the southern hemisphere is in winter with the south pole tilted away from the Sun. Six months, or half an Earth orbit later, the northern hemisphere points away from the Sun, and we observe winter in the northern hemisphere and summer in the southern hemisphere.
There are special dates on our calendars to precisely mark the key locations in the orbit of the Earth which coincide with the seasonal changes. The summer solstice occurs around June 21 and marks the beginning of summer in the northern hemisphere. It is also the longest day of the year. At the summer solstice the north pole is tilted as much toward the Sun as it ever gets. The winter solstice occurs around December 21 and is the beginning of winter in the northern hemisphere. It is also the shortest day of the year. The winter solstice is when the north pole is tilted as far away from the Sun as it ever gets.
It is easy to think of the equinoxes of spring and autumn as the midway points between the solstices. The name equinox implies equal nighttime and daytime. At the moment of equinox, the poles are pointing neither toward nor away from the Sun. This is because the spin axis of the Earth is at right angles to the Sun’s illumination during an equinox. The vernal equinox occurs around March 21 and marks the start of spring in the northern hemisphere. The autumnal equinox occurs around September 21 and is the beginning of autumn.