Farther vs. Further

As they relate to each other:

Farther defines distance. It compares how far apart two things are. Most often the two things being compared are a person and an object or two people.

• Example: After walking in the wrong direction for 20 minutes, John is now farther away from the bank than his friends.
  

Further means to a greater degree, a greater extent, or in addition (moreover). It is most often used when there is no notion of distance.

• Example: I do not want to discuss this matter any further.

When discussing a metaphorical distance, both farther and further are used interchangeably.

Memorization tip: The word 'far' is in farther. Far always relates to distance. If that is the case, use farther.