Farther and further are comparative adverbs or adjectives. They are the irregular comparative forms of far. We use them to talk about distance. There is no difference in meaning between them.
Although some usage guides insist that only farther should be used for physical distance (We walked farther than we planned), farther and further have been used interchangeably throughout much of their histories. However, only further is used in the adverbial sense "moreover'' (Further, you hurt my feelings) and in the adjectival senses "more extended'' (no further comment) and "additional'' (Further bulletins came in).