The ampersand ("&") actually began in the days of handwritten manuscripts as a ligature of the Latin word "et" (meaning "and")  and the letter "s" (which then often took a form resembling an "f"). 

Writing books by hand was a painstaking process, and so ligatures  were often used to save time and effort (not to mention valuable In modern typography, many of these old ligatures have largely fallen out of use,
at least partly because they make it more difficult to divide words between lines. 
 Some publishers still use certain  vowel ligatures (such as "æ") for stylistic effect,
since English words aren't generally divided between vowel combinations of this sort;
this practice, however, is increasingly rare. 
 The ampersand, on the other hand, is still quite widely used in certain contexts (such as business names and informal writing), perhaps due to the fact that it still saves significant space and effort, even by modern standards (whereas a ligature such as "æ" barely saves space, and often requires more effort to reproduce on today's keyboards than the simple letter sequence "ae").  It's also exempt from the end-of-line division problem, since the monosyllabic "and" is virtually never broken at the end of a line anyway.


Courtesy of 
http://www.Merriam-Webster.com http://www.WordCentral.com

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