I know I learned the difference between the usage of farther and further in school, but I can never remember where each one should be used. Can someone help me out here?
Farther is used for distance. Further is used for all other purposes.
English originally had "further" as the comparative form of "fore" and "farrer" as the comparative of "far." "Farther" came later, presumably as a back-formation, and has been interchangeable with "further" throughout most of the history of the written language. The differentiation between "further" and "farther" seems to have been invented ca. 1900 by grammarians. It's not a distinction that has ever been widely accepted, and it's not worth fighting to maintain it because it doesn't enrich the language. Because it has never been a natural part of the language, there is not widespread agreement on what criterion should be used to distinguish them: figurative versus literal, or addition versus distance. There is no clear evidence that the best writers have systematically observed the distinction. "Further" can be used literally:
"Farther" can be used figuratively:
To many people's ears, including mine, the figurative use of "farther" sometimes sounds a little off ("I could use some farther income."), but that doesn't mean it's incorrect. Literal "further" may be more common in British English than in American English.
There is no value in trying to fight for the distinction, since it doesn't help us to make shades of meaning; the meaning is either literal or figurative based on the context, and this distinction can't be blurred or confused by which form of the word we choose.
Even relative prescriptivists such as Fowler (Fowler's Modern English Usage, 2e) don't advocate for maintaining the difference between the words, and the OED doesn't find much historical support for it.
The following points may be helpful for nonnative speakers:
For all of these reasons, nonnative speakers are safer saying "further" when in doubt. Substituting "further" for "farther" is never incorrect, and many native speakers use "further" exclusively. Fowler speculates that "further" is eventually going to displace "farther" completely.
further refers to degree
farther describes distance
Haven't you seen Finding Forrester? :)
"Farther" seems to be one of the many differences of American English. It's not really used in England and the rest of the UK. I have only heard it used by Americans, on television programmes. It is explained further in these links http://english-usage-mcallister.blogspot.co.uk/2006/06/farther-or-further.html and http://random-idea-english.blogspot.co.uk/2011/10/q-further-or-farther-british.html
It sounds weird from an English and British perspective.