The only time someone told me my grammar sucks was when I told him to get the first cab going home because his house was farther than mine. That led to an argument whether the correct word was farther or further. To be fair, he was drunk, and I was rather under the influence of alcohol—fine, I was drunk too. The fact that I remembered that despite being drunk that time means I took that argument seriously. True enough, most English speakers use the words farther and further interchangeably, but there are subtle differences between the two, which is actually very straightforward.
Farther is used to express physical distance, as in “your house is farther than mine.” Further is mostly used to express figurative distance or to discuss degree or extent, as in “I wanted to explain it further, but you are rather busy.” Also, you can further, or advance, a project, but you cannot farther a project because it cannot be used as a verb. Further can also be used as an adverb, meaning, “moreover” or “additionally,” so you can say “Further, you insulted my intelligence,” but not “Farther, you insulted my intelligence.”
While the physical vs. figurative distance distinction is a general guide to good usage, it is not always adhered to in popular usage, something that you will find in the definitions of these words. At the same time, knowing the difference between good usage and popular usage will set you apart in formal settings and in the company of style-guide devotees.