User with this ability either is or can transform into a demon, an evil spirit, fallen angel or Satanic divinity. Most commonly user retains their general form but with some additions; horns, claws, fangs and tail are common, as are wings, more fit (muscular and/or larger) body and color change (red, blue or black dominate) but there are several kinds of demons who are far less humanoid, varying from strange, surreal or even absolutely horrific.
For practical reasons demons can be divided into three groups defined by what they are formed from:
Physical Demons aren't necessarily solid matter, but they are material/energy beings with relatively defined/stable form and often at least somewhat human-like mind. Most users that aren't originally of demonic stock have this variation. Note that demons of this group aren't necessarily any weaker than those of the other groups, they are simply more "solid" in mind and body.
Spiritual Demons are completely spiritual beings, and thus far less bound to the laws of physics or single form. Many of them think in ways that are only tangentially relatable to humans, and their forms are both transient and often downright surreal.
Nether Demons are often both the most powerful and ancient demons, in some ways closer to sentient concepts than anything else. As such, relating to them in any way is likely impossible.
Many Demons have different interests and/or they are Created for different purposes, so many have additional abilities, like Demons of Plagues, Wrath or other Seven Sins just to name few. For more examples, see The Lesser Key of Solomon.
Ciel Phantomhive (Black Butler) after becoming a Demon.
(from left to right) Thompson, Timber and Canterbury or simply known as The Triplets (Black Butler) are Demon siblings.
Although he appears to be human, Czar Baldy III (Bobobo-bo Bo-bobo) is actually a demon.
Porusutoroi (Bobobo-bo Bo-bobo) obtains a demon form when he slices himself across the chest.
Natsu Dragneel (Fairy Tail) after his apparent 'Death' over four hundred years ago, his older brother Zeref brought him back to life as END (Etherious Natsu Dragneel) Zeref's strongest demon out of all his creations. Though he's currently unaware of his status as an Etherious demon.
Mard Geer (Fairy Tail) After transforming into his true Etherious form.
Mirajane (Fairy Tail) uses Take Over magic to take the appearance, abilities, and powers of a demonic being which is called Satan Soul.
Neo Minerva (Fairy Tail) after becoming a Demon.
Nine Demon Gates (Fairy Tail)
Kappelmeister (Marchen Awakens Romance) is driven off the edge with Team MÄR’s interference and resorts to absorbing his own Ghost Chess henchmen into his body. After the absorption was completed, Kapellmeister obtained a demon-like form.
The ÄRM, Allu Majuu (Marchen Awakens Romance) is a demon taking the shape of a flaming orb chained to a halberd.
Guardian ÄRM, Tiamat (Marchen Awakens Romance) takes on the form of the demonized mother goddess of the ocean