While popularized in the world of fantasy novels and role-playing games, firewhips do actually exist. In reality they are made of braided Kevlar or other similar aramid fiber, and a metal handle. These fibers are critical because they can withstand extremely high heat and remain flexible and strong.
The firewhip is soaked in a petroleum distillate like Coleman stove fuel and ignited. When the whip is cracked the fuel spray creates a fireball.
A skilled firewhip user can throw a fireball 10-15 feet and hit a target with a high degree of accuracy. Unskilled users light themselves on fire and can potentially die horrible screaming deaths.