There is no fundamental difference save spelling.
quote from the Oxford dictionary:
Is there any difference between further and farther? In the sense ‘at, to, or by a greater distance’ they may be used interchangeably: she moved further down the train and she moved farther down the train are both correct. However further is a much commoner word, and in addition it is used in certain abstract contexts, for example in references to time, in which it would be unusual to substitute farther, e.g. have you anything further to say?; without further delay.