Farther: the comparative form of the adjective and adverb far; is often followed by than.
Further: may be used in the adverbial sense of “moreover,” as in Further, you hurt my feelings; in the adjectival sense of “more extended,” as in no further comment, and of “additional,” Further bulletins came in. Also, only further can act as a verb, as in He wants to further his career.
Note: Some usage guides insist that only farther describes physical distance (We walked farther than we planned). But farther and further have been used interchangeably throughout much of their histories.
“Parcells Decides to Return, but Heads Farther South” —New York Times Headline December 20, 2007
“The Primary, Upon Further Dissection” —New York Times Headline January 11, 2008
Grammar.com’s section on Problem Words has a similar discussion on further and farther. Click here for that discssion.