Demons appear as smoke in their incorporeal form, they manifest by possessing humans, alive or recently deceased, via demonic possession. Acheri Demons are the only exception, as they are able to manifest from their smoke form into that of a little girl. When an exorcism is performed, this forces the demon to leave its vessel or meatsuit, and the smoke is usually seen pouring out of the person's mouth. In nearly all cases the smoke is black, except for Crowley, whose smoke is red.
Abaddon possesses the ability to partially enter a person, by sending a piece of her "smoke" form into the person's body to read their thoughts.
Presumably, demons cannot be killed in their smoke form, as Castiel was shown forcing Ellsworth back into his vessel before killing him.
Before demons appeared in smoke form, they were initially shown leaving a vessel in the form of a swarm of insects, as shown in Phantom Traveler.
Presumably, a demon's smoke form is the state of a soul after it has been tortured into becoming a demon, showing it to be corrupted. As demons are essentially spirits or souls, as Bobby Singer stated in Weekend at Bobby's, this may be the case.
In We Happy Few, a weakened Amara is physically attacked by a massive swarm of demon smoke. The combined demon smoke is able to lift her into the air and a strike from Crowley in his smoke form knocks her to the ground and causes her injury.
In Damaged Goods, after being released from an Enochian Puzzle Box, Abraxas' demon smoke is contained by a devil's trap that the smoke was released into. The demon smoke visibly hits a barrier when it tries to escape the devil's trap, forcing it to take a vessel left in the trap with it instead.
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