[ adverb ] to the greatest distance in space or time (`farthest' is used more often than `furthest' in this physical sense)
"see who could jump the farthest" "chose the farthest seat from the door" "he swam the furthest"
[ adverb ] to the greatest degree or extent or most advanced stage (`furthest' is used more often than `farthest' in this abstract sense)
"went the furthest of all the children in her education" "furthest removed from reality" "she goes farthest in helping us"
[ adjective ] comparatives of "far"; most remote in space or time or order
"had traveled to the farthest frontier" "don't go beyond the farthermost (or furthermost) tree" "explored the furthest reaches of space" "the utmost tip of the peninsula"