What is the difference between farther and further?

Which part of the dictionary entry did you use to find the answer?

Farther for physical distance and further for metaphorical or figurative distance.

The quick and dirty tip is to use "farther" for physical distance and "further" for metaphorical, or figurative, distance. It's easy to remember because "farther" has the word "far" in it, and "far" obviously relates to physical distance.