Farthest means the most distant in space or time. When you throw a ball as hard as you can, the farthest throw is the one that travels the greatest distance. If you're planning a family road trip, the farthest destination might be the one that's 500 miles away instead of 50.

You can use farthest for physical distance: Neptune is the farthest planet from the Sun (now that Pluto has been reclassified). You can also use it for progress or extent: the runner who gets farthest in a race goes the greatest distance before stopping.

The word furthest means almost the same thing and people often use them interchangeably. Some careful writers prefer farthest for actual physical distance (“the farthest star”) and furthest for figurative or abstract distance (“the furthest thing from my mind”). But in everyday speech, either word works fine.

When you're describing the extreme end of something in terms of distance, farthest is your word. It marks the outer limit, the point beyond which you cannot go: “We hiked to the farthest edge of the canyon and looked out over the valley below.”