Refer to figure.
All planetary orbits, including the Earth's, are elliptical with the Sun at one focus. The Earth moves slightly closer and further away from the Sun during its yearly orbit.

The point where the Sun is furthest from the Earth is Aphelion and where it is closest, Perihelion. The highest speed of the Earth in its orbit is at perihelion, the lowest at aphelion. Perihelion occurs in early January and Aphelion in early July.

This must not be mistaken for the cause of the seasons, as actually the Earth is farthest from the Sun in early July, and therefore receives slightly less energy per day, and yet the Northern Hemisphere is in the height of Summer. The seasons are instead set by the inclination (tilt) of the Earth.