The ampersand is a character used in many Western languages to denote the word and. It is an extremely old character, and its ancestor can be traced back to the first century, where it was used in Old Roman. The symbol itself has changed a great deal over the years, but similarities can still be seen between the ancient version and the modern version. Many typefaces treat the ampersand as a very graphic character, and so add a great deal of flourish and style to it. As a character entity in codesets like HTML, the ampersand is represented by &, or by &.
In Old Roman the word et, meaning and was often written in a very close form of cursive, forming a ligature. As Roman Cursive evolved, this basic et became more stylized, and began to appear more like a modern ampersand. As the Latin script evolved, most other letters lost their ligature, but the ampersand retained it, and became even more stylized, making it look simply like a graphical character, as opposed to a set of letters. In fact, the ampersand was for many years treated as an individual letter in the alphabet, as the twenty-seventh letter.
There was an old tradition of saying per se, meaning simply by itself before any single letter that stood as a word by itself, for example per se I or per se A. Ampersand was treated in the same way, described as per se and, which meant the alphabet ended: X, Y, Z, and per se and. Over time this became slurred into andperseand, and eventually just became the ampersand we all know.
There is a folk etymology about the origin of the word ampersand, as well, although it appears to have no validity. Some people hold that the symbol was widely used in publications by Andre-Marie Ampere, and so it started being called Ampere’s and, which eventually got shortened to ampersand. In fact, the term had been around for quite some time before Ampere was born, so this etymology is almost certainly false.
Generally, style guides agree that the ampersand should not be used in normal writing. Historically speaking, however, it was used commonly in text, and when reading old documents the word and is generally replaced by the ampersand in all situations. In the modern world, however, the only place one tends to see it outside of casual writing between acquaintances is in product names or the names of firms and partnerships, such as Mitchell & McDonald.
In some cases, the choice between using the word and and the ampersand actually indicates something. For example, in movie credits an ampersand indicates that two people worked closely together, as in two writers who collaborated on a script. The word and spelled out indicates that though they may have both worked on a script, they likely did not work directly with one another.
In more archaic uses, the ampersand may also have been used in conjunction with other letters or words, in a similar style to modern texting speech. The best example of this is the 18th and 19th century contraction of et cetera. In modern writing one often sees this shortened to simply etc., but in older writing one would often see &c. as a further contraction of this, with the ampersand representing the word et.