With some varyingly enthusiastic contributions from the rest of the class, the following picture emerges:
Fire magic, Katherine's affinity, deals with fire, emotions, and energy transfer. Professions this is useful for include firefighters and therapists; energy transference is mostly the realm of theorists at the moment but researchers are claiming it's going to replace steam power. The ethics of using emotion magic in warfare are hotly debated.
Air magic deals with air, minds, and communication. Air-based communicative telepathy is used to let groups of people talk to each other without speaking out loud or even being in the same room, and illusions are a favoured tool of entertainers. The use of mind magic in warfare is very unethical and very very illegal.
Water magic deals with water, divination, and far-seeing. Past-watchers can be hired by someone investigating a crime; future-seers help to prevent crimes or disasters before they occur. Far-seeing can stretch much further than telepathy and is used for long-distance communication across continents, but it can't convey sound.
Ice magic, Temperance's affinity, deals with ice, defence, and time manipulation. It is used for preserving food or corpses, firefighting, combat, and security.
Stone magic deals with earth, metal, transmutation, and magic items. Stone mages tend to be either combat mages or manufacturers.
Nature magic deals with living things: plants, animals, and healing. Accordingly, it is used for farming and medicine. Some monarchs hire nature mages to keep them young and stop them dying of illness or accidents.
There is also Aether, which is not so much an element as a category of non-element based magic. This type of magic is all about magic itself. One example is magesight, which can be used to tell whether someone is a mage and how much mana they have.