From PathfinderWiki

The fires of Hell are a common linguistic and literary image but hardly compare to the malicious and deadly reality of actual hellfire. It burns without need of fuel, and can be found easily in water, rock or air. Unlike common flame, hellfire seems to be driven by a malicious will intent on destruction. Stinking of sulfur, its flame burns with an unholy radiance that disproportionately harms good and moral creatures. Hellfire is most commonly encountered in the layers of Phlegethon, Malebolge, Cocytus, Caina, and Nessus.1


For additional as-yet unincorporated sources about this subject, see the Meta page.