Like many letters in our current alphabet, the ampersand probably began as a convenience. The Latin word et (meaning "and") was first written as two distinct letters, but over time the 'e' and 't' were combined into a ligature of sorts. Once the ampersand was accepted as a single character, artistry took over and a more flowing design evolved. Credit for the invention of the ampersand is usually given to Marcus Tiro, who included it in a shorthand writing system he devised in 63 B.C.
By the time Charlemagne's scribes developed the Carolingian minuscule in about 775 A.D., the "et" ligature had become a standard part of their repertoire. It remained so all the way until the invention of printing in the 15th century, when the ampersand was adopted, with enthusiasm, by the first typographers. For example, Francesco Colonna's Hypnerotomachia Poliphili, printed by Aldus Manutius in 1499, used twenty-five ampersands on a single page. Why such liberal use of the ampersand? Early printers wanted to keep their work in the tradition of hand-illuminated manuscripts. And, like us, they considered the ampersand a particularly beautiful character and enjoyed using it for its aesthetic merit.
The word "ampersand" is an alteration of the phrase "et, per se and" (that is: "et by itself [means] and"), which became corrupted to "and, per se and", and finally, ampersand.