Farther refers to length or distance. It is the comparative form of the word far when referring to distance.
Further means "to a greater degree," "additional," or "additionally." It refers to time or amount. It is the comparative form of the word far when meaning "much."
Correct: London is farther north than Juneau.
(Refers to distance)
Correct: This plan requires further study.
(Meaning "additional study," refers to amount)
Correct: According to my timetable, we should be further along.
(Refers to time)
See also Irregular Comparisons.