If good-luck charms are one of the most common types of folk magic, I suspect curses are right up there with them.
This time I don’t mean profanity (though there’s a degree of overlap there). I mean actual malevolent attempts to cause someone harm by supernatural means. Sometimes people do this deliberately, out of a desire for power or revenge; other times it’s a subconscious process, the metaphysical consequence of negative emotions like anger, jealousy, or fear. The latter shows up in the Japanese concept of an ikiryō, a “living ghost” that is the projection of a person’s spirit, or in the (I think) Asante concept of witchcraft as a thing people can do without meaning to or realizing that it’s happening.
The deliberate versions are easier to talk about because they are, for obvious reasons, more concrete. Many curses amount to codified ill-wishing, with a profoundly fuzzy boundary between religion and magic — here taking “magic” in the sense of “supernatural means not approved of by religious authorities”. Imprecatory prayer is the act of petitioning gods, saints, demons, or other such powers to bring misfortune on your enemies. In Greco-Roman times these were often written on curse tablets. Some religions think that kind of thing is totally fine; some don’t; and some just avoid commenting on the fact that praying for your own side to be victorious in battle amounts to wishing for the other side to die. (Mark Twain commented on it, though.) Is it not a curse if the negative consequences for someone else are just an ancillary effect, rather than the focus of your prayer? Is there a meaningful difference between “please, God, lead my allies to victory” and “please, God, lead my enemies to defeat?”
That’s a topic for theologians and ethicists to debate. We’ve got more than enough genuine, unquestionable examples of curses to keep us busy. Consider the poppet, aka the “voodoo doll” (which does not actually originate with that religion). Operating on the principles of sympathetic magic, which hold that an object which resembles or contains a piece of the target has a connection to that target, people all over the world have crafted little dolls and figures for supernatural use, both good and bad. Sticking pins in the doll is meant to cause illness; burning it or “drowning” it is meant to kill the target. In Tamora Pierce’s Song of the Lioness quartet, a number of poppets are tied up in a veil as a magical effect to prevent them from “seeing clearly,” i.e. realizing what the villain is up to. Fear of what sympathetic magic might do has led people to take precautions with their hair clippings, fingernails, and so forth, burning or otherwise disposing of them lest they be employed in a poppet or other curse. Execration texts leverage names for the same connection, which is why certain magico-religious traditions of both the real and fictional variety have practices designed to protect people’s names against such threats.
Other types of objects can inflict ill fortune. Maybe you craft some sort of “hex bag” and hide it in the target’s house or under their bed, or bury it under their front step. Or you bury it along a road or path you know they’ll travel, and when they cross over it, the curse will attach to them. Subtle, hidden curses of that sort are intended to be more difficult to detect and counter. On the other hand, sometimes lack of subtlety is the point: the Indigenous Australian bone-pointing practice is a very public delivery of a death curse. The Germanic “nithing pole” or niðstang involves cutting off the head of a horse and sticking it on the end of a rune-carved pole, facing toward your enemy’s house — which amounts to a neon sign announcing you wish your target ill, at least if you put the pole anywhere visible. (And in getting the Wikipedia links, I am weirdly charmed to find that some Indigenous Australians performed the bone-pointing ritual against their prime minister in 2004, and Icelanders stuck cod heads on poles in 2016 to protest their prime minister. Anyone who thinks such things are confined to the past is wrong.)
Curses can also be set up such that certain actions will trigger them, as with the protective spells sometimes placed on Egyptian tombs by priests seeking to deter grave robbers. Stealing precious objects from religious sites is in general a route to misfortune, for obvious reasons: angering a deity is rarely a good idea. Book thieves have been threatened since the days of Ashurbanipal; medieval European scribes warned them of a variety of punishments, ranging from mortal measures (excommunication, execution) to divine intervention (damnation). Breaking an oath may leave the traitor accursed in one fashion or another, which is sometimes laid out explicitly in the oath itself. People may place curses with their dying breaths, or the simple fact of their death may lay the curse — especially if the victim was the killer’s kin, such acts being deeply taboo in many cultures (cf Cain and the Greek notion of the Furies).
As with good-luck charms, there’s a psychological effect in play. The nocebo response is real; it’s the opposite of a placebo, making negative outcomes or side effects more common if you expect them. Knowing someone has cursed you may have all kinds of effects, ranging from depressing your immune system to making you more accident-prone to drawing your attention toward the bad things that happen to you, causing you to see a pattern in them where another person might see random chance. Performing a curse against someone reinforces your dislike of or opposition, which may encourage you to take other actions against them, or rally other people to your side. So even in stories not built around the assumption that things like curses have metaphysical force, the idea may still show up, because it still carries very real social weight.