Amy has it right. You use the "either... or..." construct when you are trying to emphasize that there are only two options available. Thus:
"I'm thinking of buying either a Toyota or a Nissan," implies that I've made up my mind, and am limiting my decision to those two brands only.
"I'm thinking of buying a Toyota or a Nissan," implies that these are examples of the kinds of brands I'm considering, but I'm not necessarily locked in.
Actually, either/or constructions aren't limited to only two options; you can use 'either/or' with multiple options.
According to the dictionary:
Merriam-Webster Collegiate Dictionary said:
either (conj) used as a function word before two or more coordinate words, phrases, or clauses joined usu. by 'or' to indicate that what immediately follows is the first of two or more alternatives.
I think the main distinguishing factor is that using 'either/or' emphasises that only one choice can apply, not the number from which to choose.
For example, let's take a sample (if lame) sentence.
For plot planning, you can use index cards, sticky notes, or a computer spreadsheet.
No one would intrepret this to mean that you can use only one of these tools: you could use one, two, or all three.
But if the sentence were:
For plot plannng, you can use either index cards sticky notes, or a computer spreadsheet.
that would mean you could use one and only one of the tools (which is, of course, untrue anyway
).
So, to sum up, the purpose of 'either/or' is usually to make it clear that only one of multiple options can be used.
