These days the ampersand is used mostly in business names, but is often frowned upon as an alternative to the more general and personal use of ‘and’.
The shape of “&” predates the word ampersand by 2,000 years, when Roman scribes wrote in cursive. They wrote the Latin word “et” which means “and”, then started to linked the “e” and “t” together.
Over time, as English replaced Latin as the establishment language of written communication, the combined letters came to signify the word “and” in English as well.
The word “ampersand” came hundreds of years later when “&” was included as the final letter of the English alphabet for teaching purposes. In the early 1800s, school children reciting their ABC ended the chanted alphabet with the “&”.
It would have been too confusing to say “X, Y, Z, and” So the students said, “X, Y, Z and per se and.” “Per se” means “by itself” in Latin, so the students were essentially saying, “X, Y, Z, and, by itself, and.” A contemporary reference apparently exists to the children’s habit of saying “and Percy and” which seems eminently believable.
The words of the phrase “and per se and” started to get slurred together into the single word we use today: ampersand.