Our confusion today comes from the changing of one little vowel. Is it a or is it u? Here we have two words that have similar meanings and similar spellings. They are usually used interchangeably and in all fairness you shouldn't be sent to the gallows for doing so.
We'll take the liberty of explaining things for you. With what we're calling "how's your farther?"
That said, if you do want to know the difference it's quite simple.
Further is in degree of progression. It's having progressed more in a task, it technically does not mean in terms of distance. Though that is shifting with the common usage of it to mean both.
"How is your essay coming along?"
"I'm further along than I was yesterday..."
|"...as in I have the paper."|
Farther is a degree in distance. "My house is farther away than yours". It's easier to remember as it has the word far in it. If you do get stuck, avoid using it. Further is generally more acceptable and, if you like a bit of a gamble, is more likely to be correct that farther.
|A Mini would get you farther on this road|
than a Hummer would.
Does it matter?
We like to think getting things right in terms of language is important. There's always a time and place for it and correct usage of the two words probably won't matter in the pub, but if you want to get it right then further is metaphorical progression and farther is physical progression.