Think of an ellipse as a slightly squashed circle. This means that the Earth's distance from the Sun varies throughout the year. It's closest to the Sun in early January (perihelion) and farthest away in early July (aphelion).

While the difference in distance isn't huge (about 3 million miles), it does affect the Earth's orbital speed. It moves faster when it's closer to the Sun and slower when it's farther away. This is due to Kepler's Second Law of Planetary Motion, which states that a planet sweeps out equal areas in equal times.