During the equinox (when you ignore twilight) the length of day and night is equal across the globe. After today, nights will become longer than days in the southern hemisphere and days will become longer than nights in the northern hemisphere.
The reason we have seasons is because of the earth’s tilt. The earth’s axis isn’t vertical (in relation to its orbital plane) but rather on it’s side by 23.5º. Because of the earth’s orbit around the sun during winter, the northern hemisphere tilts away from the sun which allows temperature to drop, and vice versa in the summer.
Most of the planets in our solar system are tilted and have “seasons”. Mars has a tilt around 25º similar to us while Venus has a tilt of about 177º (meaning it rotates basically upside down… if you took physics in high school or college think of the “right hand rule”). Mercury has a tilt of less than one degree (meaning it rotates nearly vertically) with basically no change from their winter to their spring. Not to mention the fact it’s always hot as hell on Mercury.
One interesting nugget is that earth’s seasons are not of equal length. Winter only lasts for 89 days (December solstice through Vernal equinox) while summer lasts 94 days (Summer solstice through Autumnal equinox). The reason for this is because the earth rotates around the sun in an eliptical orbit and not in a circular orbit. When the earth is closest to the sun (perihelion) it moves faster than when it’s farthest from the sun (aphelion). Since perihelion is in early January it makes winter go by just a little bit faster and during aphelion (in June) the earth is moving more slowly. Next year when you’re fed up with winter you can thank Kepler’s Second Law (you can read the proof on wikipedia!) for keeping winter 5 days shorter than summer!