Axial Tilt
Axial tilt is the angle between a planet rotation axis and the perpendicular to its orbit; Earth is tilted 23.5 degrees, which causes the seasons.
Definition
Axial tilt is how much a planet's spinning axis leans over instead of standing straight up; Earth's axis is tilted by $23.5$ degrees, and this tilt is the reason we have seasons. Formally, axial tilt, or obliquity, is the angle between a planet's rotation axis and a line perpendicular to its orbital plane. Because the axis keeps pointing the same direction in space, each hemisphere leans toward the Sun for half the year and away for the other half, which changes how directly sunlight strikes the ground and how many hours of daylight there are. At latitude $\phi$ the noon solar elevation ranges between $90^\circ - \phi + 23.5^\circ$ and $90^\circ - \phi - 23.5^\circ$ over the year, and the energy per square meter scales with the sine of that elevation; obliquity also defines the tropics at $\pm 23.5^\circ$ and the polar circles at $\pm 66.5^\circ$. Earth's obliquity oscillates between about $22.1^\circ$ and $24.5^\circ$ with a $41{,}000$-year period, one of the Milankovitch cycles that pace ice ages, and other planets differ widely: Mars $25^\circ$, Jupiter $3^\circ$, Uranus $98^\circ$.
Example
In June the northern half of Earth leans toward the Sun, so North Carolina gets long, hot days; in December the northern half leans away, so days are short and cold. At noon on the June solstice the Sun stands directly overhead at $23.5^\circ$ N, the Tropic of Cancer, and on the December solstice it stands overhead at $23.5^\circ$ S, the Tropic of Capricorn; those two latitudes exist because of the tilt. At Charlotte ($35^\circ$ N) the noon Sun reaches $90 - 35 + 23.5 = 78.5^\circ$ above the horizon on the June solstice and only $90 - 35 - 23.5 = 31.5^\circ$ on the December solstice. Sunlight at $31.5^\circ$ is spread over about $1.9$ times more ground than sunlight at $78.5^\circ$.
Key Insight
Seasons are not caused by Earth being closer to or farther from the Sun; Earth is actually closest to the Sun in early January, in the middle of our winter. The two hemispheres always have opposite seasons, so when it is summer in Charlotte it is winter in Australia, because only one hemisphere can lean toward the Sun at a time. Earth's distance from the Sun varies only about $3\%$ over the year, which changes incoming sunlight by about $7\%$. The tilt changes noon sun intensity at mid-latitudes by nearly a factor of two and doubles the length of the day, so tilt dominates distance completely.