RPGs

There is a lot of confusion about the three D's of 'evil' creatures: devils, daemons and demons. This text explains which is what.

Devils

Devils are spirit creatures who are quite varied but form a distinct category because they share a common attribute: they are all evil. Evil that is in the sense that offends the culture that believes in them, which can be quite variable. For example some sexual behaviors can be sins in one culture but perfectly acceptable in others. But in general devils being evil means that they are cruel and uncaring about others. Often devils are servant of an evil deity, or the evil deity in monotheistic religions, who is simply more powerful than the rest. In many mythologies, devils are not simply evil but actively work evil, by luring people into evil deeds and/or misfortune. Often but not always their native home is some kind of hell, which is a nasty place where their victims suffer torments.

Daemons

The word daemon is not pronounced as 'deemon' (IPA /di:mən/) but as 'daimon' (IPA /dʌɪməʊn/). Daemons are also a type of spirit, of Greek origin. Their original name is δαίμων. They occupy a kind of middle ground between humans and deities. They represent human emotions, states of mind or being, qualities and more. Some examples are Eros (love), Hebe (youth), Nike (victory), Phobos (fear), Plutus (wealth) and Thanatos (death). Despite their power, they are seldom acting as characters in stories, just doing their thing in the background.
Some daemons are evil, others are not. All daemons being evil is a school of thinking that seems to have started with Plato and was expanded by the Christian Catholic church. The 'original' Homeric daemons, like many Greek mythological beings, do not occupy themselves with the dualistic concept good-versus-evil at all.

Demons

Demons are neither devils nor daemons. Technically, a demon is a being from another world. On the lower end of the scale it can be a foreigner visiting another land, unfamiliar with its climate, ecology, languages, customs and such. Aliens from another planet, with totally different biology and/or mindset, can be considered demons too. Examples are a husky in the tropics, a city dude among reindeer herders, an astronaut on Mars. The most extreme form of demon is a being from another universe who is physically incompatible with the world, which is science fiction, though more fiction than science.
Like daemons, demons are not evil per se. However most of the time they are quite out of place, which causes many problems. Either the demon adapts (if at all possible), or he/she/it suffers and may even die, or the world around the demon suffers harm. These frictions may often be labeled as evil, though the alien seldom is intent on causing misery.