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 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.
A notable consequence of the world's development of magic, unique even among the magic-oriented worlds, is that there exist detailed classes and categories of magic-user. In 12A, words like "witch", "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 accreditations, certifications, etc. recognising a person's ability (similar to degrees and diplomas for more mundane subjects in our world).
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.