WordWeb Online

1. 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"
   - furthest
    
2. To the greatest degree or extent or most advanced stage ('furthest' is used more often than 'farthest' in this abstract sense)
   "she goes farthest in helping us"
   - furthest

1. (comparatives of 'far') most remote in space or time or order
   "had travelled to the farthest frontier"; "don't go beyond the farthest tree"
   - farthermost, furthermost, furthest, utmost, uttermost

See also: far