Flash Fuels - include dried grass, twigs, leaves, and light branches.  They burn very easily, and fire spread quickly within them.  Once burning, they start larger fires in heavy fuels, which become horribly dangerous.

Heavy Fuels - are tree stumps, heavy limbs, trunks and the build up of pine needles and other packed down dead organic matter on forest floor. These fuels take lots of heat to ignite, but are very dangerous once lit.  They are very hard to extinguish.

Green fuels - are anything living.  They are usually not consider flammable, but when a fire gets hot enough, leaves can dry out and can continue to burn.  Green fuels are also dangerous for the chemicals they might carry:  evergreens, like pine, cedar, fir, spruce, and eucalyptus all contain oils that can explode when ignited.