Further and farther can be used interchangeably in most cases, although it's more correct to use farther when you're talking about physical distance, as in "the birds flew farther away from me," and further to mean an abstract distance, when two things are moving apart in terms of ideals, agreement, or thoughts. There's also a verb form of further that means "to promote the growth or progress of," which you might use in a sentence like, "These new gigs should help him further his success as a rock musician."