If you want the dictionary definition, just double click on any word.
Grammarians get their knickers in a twist over this one, so I'm going with Michael Swan, because he agrees with my thoughts on the matter.
We use farther and further when we are talking about distance. In this context there is no difference in meaning.
For example:
The shops are farther / further than he said they were.
I was so tired I could go no farther / further.
When we are talking about quantity and degree, we use further (but not farther).
For example:
"Further to my previous correspondence, we may have to do some further research on this."
"Everyone keeps telling me I need to further my career."
!Note - In BrE, you are pretty safe using "further", as it covers both cases, and, as a bonus, people won't think you're talking about your father.