Farther is used in relation to physical distance; it means "at or to a greater distance" whereas "further" means "to a greater degree". An easy way to remember is to use the mnemonic "In a galaxy far, farther away..", which will remind you to use farther only in the context of distance.
edit Usage of Farther vs. Further
The word "farther" can only be used in the context of distance. For example "How much farther do we have to go before we can stop for food?" Although some usage guides insist that only farther should be used for physical distance, both farther and further have been used interchangeably throughout much of their histories.
However, only "further" is used in other contexts like:
- the adverbial sense "moreover" ("Further, you hurt my feelings."), and
- as an adjective meaning "more extended" ("I have no further comment.") or "additional" ("Further bulletins came in when people were eagerly awaiting them.").
edit More examples
Here are some more examples:
- BP failed to contain the oil spill to avoid further damage.
- The Greek debt crisis made its socioeconomic impact much farther and spread to all of Western Europe.
- A CT Scan after her accident showed that metal had not gone farther than the flesh and into the bone.
- Lady Gaga's dress was atrocious. Further, her red shoes failed to match her black dress.