Fire and flames are a terrain effect. Your character, monsters and objects will take damage from Burning when on fire.
All gasses but steam, darkness, and healing spores will burn, and Explosive gas explodes.
There are many types of terrain that can burn, including the carpet of poisonous tendrils created by a Potion of Creeping Death. This can make it useful as an improvised fuse.
The player has three primary ways of setting fire directly: Potions of Incineration, a Staves of Firebolt, and Incendiary Darts.
There are also several ever-burning monsters in the game, including the Salamander and Will-o-the-wisp.