Either/or means "one or the other (of two)." Its usage, versus the simple or structure, is often for emphatic purposes, sometimes intending to emphasize that only one option is possible, or to emphasize that there are only two options. Its use in a sentence lets the reader/listener know in advance that a list of two or more possibilities will be given.
Its negative is neither/nor, meaning "none of the two".

Pronunciation

Either has two main different pronunciations in modern English.

An Ira Gershwin

Ira Gershwin

Ira Gershwin was an American lyricist who collaborated with his younger brother, composer George Gershwin, to create some of the most memorable songs of the 20th century....

 song, Let's Call the Whole Thing Off, opens with the words "You say ee-ther and I say eye-ther", and concerns a couple who lament the strain put on their relationship by pronunciation differences.

The 'ee-ther' pronunciation forms with the word 'ether' one of the few minimal pair

Minimal pair

In phonology, minimal pairs are pairs of words or phrases in a particular language, which differ in only one phonological element, such as a phone, phoneme, toneme or chroneme and have distinct meanings...

s demonstrating that the difference between the voiced dental fricative

Voiced dental fricative

The voiced dental non-sibilant fricative is a type of consonantal sound, used in some spoken languages. The symbol in the International Phonetic Alphabet that represents this sound, eth, is . The symbol was taken from the Old English letter eth, which could stand for either a voiced or unvoiced...

 and the unvoiced dental fricative is phonemic in English.