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.
Note, however, that only further may be used in the adverbial sense of “moreover” (Further, you hurt my feelings) and in the adjectival sense of “more extended” (no further comment) and of “additional” (Further bulletins came in). Also, only further can act as a verb (He wants to further his career).
The word farther is the comparative form of the adjective and adverb: far.
Example: He wanted to further his education, but when the local school closed, he was forced to drive 10 miles farther to get to class every day.