Like all planets, the Earth obits the Sun in a near-circular, elliptical orbit. At the point where the Earth is closest to the Sun is known as the Perihelion and the point that it is furthest away from the Sun is know as the Aphelion.
The seasons of the year are not effected by the Perihelion and Aphelion as some might believe. The seasons are controlled by the relation between the orbital tilt of the Earth (about 23 degrees) and its position in its orbit. The seasons are effected by the position of the Sun in the sky and the length of the sunlit portion of the day. This is why the Southern Hemisphere of the Earth has the opposite seasons to the Northern Hemisphere. For example: The Southern Summer occurs during the Northern Winter.
The Perihelion occurs when the Earth is closest to the Sun in its orbit.
The Aphelion occurs when the Earth is furthest from the Sun in its orbit.
An Equinox occurs when the daylight and night portions of the day are exactly equal. This occurs when the Sun is perpendicular to the Earth’s tilt. Equinoxes occur twice a year and mark the beginning of the Spring and Autumn seasons.
The Vernal Equinox marks the beginning of Spring.
The Autumnal Equinox marks the beginning of Autumn.
Solstices occur when the Sun is aligned to the tilt of the Earth. The Hemisphere tilted toward the Sun at that point starts its Summer. The Hemisphere tilted from the Sun starts its Winter.
The Winter Solstice occurs when the Sun is aligned to the tilt of the Earth in the Hemisphere tilted away from the Sun. This marks the beginning of Winter for the hemisphere and is the day with the least amount of daylight (The Longest Night).
The Summer Solstice occurs when the Sun is aligned to the tilt of the Earth in the Hemisphere tilted toward from the Sun. This marks the beginning of Summer for the hemisphere and is the day with the most amount of daylight (The Shortest Night).