From the 12th to the 15th century it was hardly used, but enjoyed a resurgence in popularity after printing became established. Then it fell out of fashion again in the 1960s/70s but again had a resurgence; it is now an integral part of many business names, and has an important role in html writing. The ampersand (&), a mixture of the letters E and T, was initially conceived as a shorthand way of writing the Latin et, and. In its history it has often been less popular than the Tironian note for et, which is older than the ampersand, having been created in the 1st century BC by Tiro, a freed slave of Cicero.
Radio 4 had an interesting programme devoted to the ampersand yesterday (listen here for another week). The reporter Alastair Sooke visited a punchcutter in Paris, who made punches for traditional printing machines. It was the Parisian printer Garamond who was partly responsible for the popularity of the ampersand, because he designed a particularly elegant and beautiful one (see examples here). The 18th-century English designer of typefaces, William Caslon, also designed an influential ampersand (see here).
The word ampersand only came into being in the 19th century. At the time the ampersand was included on alphabet charts. When people recited the alphabet they would say X, Y, Z "and per se and", per se meaning 'by itself' and this became shortened to ampersand.