It differs from World 12B (our world) in that its inhabitants have science and technologies based on magic - or, as most of the Related Worlds would have it, in that the inhabitants of 12B turned away from magic and developed materialistic technology. (Most of the Related Worlds are magic-oriented, like 12A.) This occurred at some point during the Fourteenth Century, and although 12A still resembles our world in many ways relating to events before that time (the landmasses are the same, for instance, and both worlds have Christianity and The Arabian Nights and "Sleeping Beauty"), much of its subsequent history is different.
Subjects covered in primary school history classes include the reign of King Canute, the Battle of Agincourt (which was won by the French; Richard II was king of England at the time, and for some time afterward), and the part played by magic in the Wars of the Roses (not a large part, but presumably in some way a significant one).
The continent on the western shore of the Atlantic Ocean is called Atlantis, and is dominated by the Incas.
Magic-users are remarkably common in World 12A, even by the standards of other worlds where magic is an everyday part of life. Notably, and perhaps uniquely, there exist detailed classes and categories of magic-user. In 12A, words like "witch", "necromancer", "wizard", "magician", "sorceror", and "enchanter" are precise terms, referring to the practicioners of distinctly different forms of magic. There is also a set of official qualifications recognising a person's ability (similar to degrees and diplomas for more mundane subjects in our world); Certified is the lowest, and Accredited is not much better. Children can have magic lessons, with graded exams, much like music lessons in our world; there are at least four Elementry Grades, and then some number of Advanced Grades, overseen by an Examining Board.
Mr Henry Nostrum's name is followed by the letters "A.R.C.M", which might mean something like "Associate of the Royal College of Magic".
Some magicians are medical specialists, although it's not clear whether all medical specialists are magicians. Spells are used for disinfecting medical surfaces and anaesthesia, and are completely ineffective against measles.
There are laws governing the allowable use of magic, which in England are overseen by a Ministry. The Minister may himself be only a minor magic-user; the task of enforcing the laws belongs to Chrestomanci, who is employed by the Ministry for this purpose.
Standard magical reference works include Moore's Almanac and Prynne's List.
In England, the Prince of Wales was married in the mid-1970s, in the year before the main part of Charmed Life takes place. His father is King Charles the Seventh.