Either is used when you are making a choice between two nouns. For instance "I want either a banana or an orange" would mean you want a banana or an orange but not both.
Further is used to describe a greater or larger distance. Farther is also used this way (for example: "Dallas is farther away from New York than is Philadelphia" which means that the distance between Dallas and New York is larger than the distance between New York and Philadelphia). Further isn't used for physical distance but for a metaphorical distance. You could ask, "before I go any further in my speech, are there any questions?" You're asking if anyone wants to ask you something before you keep going with the speech, or move a greater distance into it if it were something you could travel through.
