If you are talking about distance, the two words can be used synonymously.
Example: We need to drive ten miles further/farther to reach our destination. * The injured man walked a little further/farther along the path before he found someone who could help him. * They tell me that Vijayawada is further/farther away than Hyderabad.
But in some contexts there is a difference between ‘farther’ and ‘further’.
While the word `further’ can be used to mean `in addition’ or `more’, you cannot use `farther’ in such contexts.
Example: If you wish to have further discussions, please meet me in my office. * The floods caused further damage to the old bridge. * I don’t think we should discuss this matter any further.