both words refer  to great distance. Though, further is used in a figuritive or metphorical sense, and farther is used to express physical distance. 

Example: "I have no further questions." 

                 "I will drive farther south."

another example: "This requires further study."

                                "Farther and farther he rode away." 

As you've maybe guessed, there's a complicated answer to this question and a simple one:

1. The complicated answer

Follow the rules, as explained above, about 'further' in abstract, metaphorical and figurative contexts and 'farther' for physical, concrete and literal ones.

2  The simple answer

Just use 'further' - it'll always be right. Forget about 'farther'.

NB If anyone insists that you have to use 'farther', just tell them you're doing things the British way.

Bummer. I've already memorized "I will drive farther south", stored it and locked it for good somewhere in the farthest (see?) corners of my mind. I'm doomed to study American English. Wait... I AM studying American English. Woo-hoo! My day is made xD.

By the way "I will drive farther(further) to the south" and "I will drive farther(further) south. Is there any particular difference between those in meaning, or are those just 2 ways to say exactly the same thing, hm?