Most planets in our solar system, and indeed most planets in the universe, have elliptical orbits. This means their path around the Sun is not a perfect circle, but rather an oval shape.

While some planets, like Venus, have orbits that are very close to circular, most have a slightly more elliptical path. The Sun sits at one focus of the ellipse, and the planet travels faster when it's closer to the Sun and slower when it's farther away.