Fans have been introduced to a variety of races throughout Dragon Ball's run. However, unless they've played the games, one race has remained rather mysterious: the Demons. This is mostly due to the way the word is used to describe various types of creatures throughout the series, mostly due to the way certain words are translated from Japanese into English. So, what are Dragon Ball's Demons, and where do they come from?
The first Demon to be introduced in the series was Demon King Piccolo of the Demon Clan back in the original run of Dragon Ball. This Piccolo was the one that a young Goku fought and killed, and the father that produced the egg the current Piccolo Jr. was born from. However, as we would later find out during the Freiza Saga, Piccolo is actually Namekian from the planet Namek. So while he was king of the Demon Clan, he wasn't part of the Demon race.
His other children, Drum, Tamborine and Symbol, also don't resemble Namekians, rather, they are actual members of the Demon Clan. YouTuber Geekdom101 has a video that goes into detail about how this is possible. He also goes on to explain that King Piccolo could control how his children looked, so that's why the three brothers look more like Demons and not Namekians, like Piccolo Jr.
The next time we hear about Demons in Dragon Ball, aside from Garlic Jr. in the filler movies, is during the Majin Buu Saga. Majin, directly translated into English, means "demon person," so the Majin are going to be the closest thing to our concept of what demons are in the franchise. Originally, Bibidi created Buu and he was the only Majin to exist. This has since been changed, but Buu is still not a Demon in the sense that he did not come from the Demon Realm -- he actually existed before it came to be, based on what creator Akira Toriyama has explained.
The first Demon from the Demon Realm we actually see is Dabura. He is stated to be a King of the Demon Realm, but he is not the only one to hold this title. While we never see another character from the Demon Realm in the anime series, more are introduced in Dragon Ball Heroes and in Dragon Ball: Sleeping Princess in Devil's Castle. The only other time we hear about the Demon Realm in the anime series is during an anime-only filler episode called "Goku Goes to Demon Land," in which Goku must rescue Princess Misa from Shula, a demon martial arts master.
The Demon Realm Race only exists in Dragon Ball video games, wherein they are shown to be skilled in magic. They usually have pointy ears and skin of various colors, and some have the ability to transform into various forms, such as the Demon-Saiyan hybrid Mira being able to achieve Super Saiyan.
Another character, Towa, is shown to be able to achieve a Dark Form. Others are able to achieve Demon God Form. Due to their Demonic nature, they excel in fighting of all types, be it magic, swordsmanship or martial arts. The games also make it known -- without outright stating -- that they are considered Demon Realm Core People, the same race as the Kais. Knowing this, the resemblance between the Kais and the Demons is more noticeable, as they all share similar physical traits like pointy ears, white hair and skin tones. Several Demons are shown to own Potaras as well, which are only given to Supreme Kais.
Ultimately, "Demon" seems to be a title that is thrown around to associate characters with the concept of evil. For instance, characters from the Demon Realm are known as the "Demon Realm Race." Devilman, a character from the Fortuneteller Baba Saga of Dragon Ball, wasn't a Demon, but from the race of Devils from Hell. They appear to not be Demons either. Evil aliens from all kinds of races have been called Demons, as well.
All in all, it looks like there isn't a pure Demon race in the way that there's a Saiyan race or Human race. It's just a catch-all for evil entities. Despite this, the Demon Realm race seems to be the true "Demons" in the series.