A supernatural and very versatile fire most famously associated with the Ghost Rider, which can sear the soul as well as burn the flesh. It is capable of burning underwater (Ghost Rider #23), healing the host body it projects from (Blaze #1) and hurt other Hellfire generators. It has generally been described as hot, but has also been recounted by Water Wizard (Ghost Rider #23), Thor (The Avengers #214), and The Skin-Bender (Ghost Rider #35) as cold and bitter while burning the soul. This is most likely because Hellfire burns at the will (intentional or unintentional) of it's host. When Michael Badilino, the original Vengeance, sells Johnny's old Hellfire shotgun to Kowalski, he attests Hellfire can kill anything (Ghost Rider #30). In Ghost Rider #21 - Hell-Bent & Heaven-Bound, Johnny Blaze reveals the flames only burn the wicked.
Hellfire also apparently protects the host body from molecular transformation. When The Skin-Bender, a woman with the power to transform the body on a molecular level into anything her mind can conjure, tries to alter and perverse the flesh of Johnny Blaze, she states his flesh denies him, that he has fire within, and that she is unable to change his form because he has already been changed (Ghost Rider #35 Trials And Tribulations).
With the help of a depowered Witch Woman, Johnny Blaze learned and managed to produce a flaming motorcycle constructed entirely out of Hellfire. He asserts the feat required a high level of concentration (Ghost Rider #3 - Wheels On Fire). The construct would later need to be reabsorbed due to the fact he cannot ride the flaming motorcycle in human form. Alejandra accomplishes a similar feat with a bike handle given to her by an undead character referring to himself as The Seeker and later manages to create her signature kama/scythe weapon out of Hellfire (Ghost Rider #1 - Give Up The Ghost).
The Ghost Rider can imbue any tool it desires with Hellfire, such as rides or weapons, enhancing their effectivity and giving them the same mystical properties. An example of this can be seen when a depowered Johnny Blaze uses his signature Hellfire Shotgun to access some of his former Ghost Rider powers, like repairing and transforming his broken bike into a Hellfire blazing ride (Ghost Rider/Blaze: Spirits of Vengeance #11).
The color of the flame; while its default appears to be no different from ordinary Earth fire, appears to be subjective based on the will of its host body. Under the empowerment of the Dark born Arch-Angel Zadkiel, Danny Ketch possessed a blue flame (Ghost Rider: Danny Ketch #5). When Johnny Blaze joined Red Hulk's Thunderbolts team, he changed his flame to a deep red color (Thunderbolts #22). The 2nd Vengeance, Kowalski, has a green flame (Ghost Rider #32 / Secret Avengers #21.1).