The tilt of the Earth's axis is the reason we experience seasons. Due to the tilt, the Sun hits different parts of the Earth with various intensities throughout the year. The Northern Hemisphere experiences summer when the North Pole is tilted closest to the Sun, and winter when the North Pole tilts away from the Sun. This is why the Southern Hemisphere experiences winter while the Northern Hemisphere experiences summer.
A lot of people think that during summer the Earth is closer to the sun and that's why it's warmer, and in winter it's further away and that's why the temperatures drop. However, this is not correct. Earth does not orbit in a flawless circle. Instead, its orbit is lop-sided. For part of its annual orbit, it is closer to the Sun than at other times. During the time the Earth and the Sun are closest to each other, the Northern Hemisphere is experiencing winter. The distance between the two is not what determines seasons. Instead, it's the axis that goes from the North Pole through the planet down to the South Pole. The Earth makes one turn around this pole daily. This rotation is why the planet experiences night and day, and even why time zones exist. The axis isn't perfectly straight, and that tilt is why the planet has different seasons.
Scientists believe that, at one time, the Earth didn't tilt. It stood straight. However, in what's known as the giant impact hypothesis, it is believed that a planet called Theia collided with Earth. The resulting collision had two lasting impacts. One, the resulting debris from the collision ended up forming Earth's moon. It also moved the Earth off its straight axis. The gravitational force of the Sun causes the different poles to try and pull closer to it.
The Earth does not orbit around the sun in a perfect circle. The perihelion, when the Earth is closest to the Sun, occurs when the two entities are 91,400,000 miles apart. The aphelion, when Earth and the Sun are at their furthest distance from each other, occurs when the two are 94,500,000 miles apart. It's a difference of more than three million miles, which considering the total distance isn't a lot. The aphelion happens in July, and the perihelion in January. So the Earth is furthest from the Sun while the Northern Hemisphere is having summer and closest during that hemisphere's winter. The reason? Because the distance between the two during orbit doesn't impact the seasons.
The heat of the sun is more directly felt by the Northern Hemisphere when the North Pole tilts toward the Sun during the summer. The sun's heat most directly hits the Northern Hemisphere during the solstice, which occurs on either June 21st or June 22nd. They are most strongly felt in the Tropic of Cancer. During summer solstice in the Northern Hemisphere, the Southern Hemisphere is experiencing winter solstice.
December 21st or 22nd is when the winter solstice occurs in the Northern Hemisphere. The tilt of the Earth's axis moves the Northern Hemisphere away from the sun. Days are shorter. These shorter days are why it's colder. The Sun's rays have less time to heat the hemisphere during the winter. While the Northern Hemisphere has winter, the Southern Hemisphere experiences summer.
The median point between the June and December solstices is called the equinox. During the equinox, the sun shines directly at the equator. Because of this, night and day are perfectly balanced. There are two equinoxes. In the Northern Hemisphere, the autumnal equinox occurs on September 22nd or 23rd, while the vernal (or spring) equinox occurs on March 21st or 22nd.