For flames and fire the term 「火」[hee] is used. For a large fire, an emergency event requiring people to evacuate and the 「消防」[shoh-boh] fire department to arrive, the term 「火事」[ka-ji] is used, typically by shouting 「火事だ！」[kah-jee-dah!] to alert those around you there is a huge fire happening. If you wish to say "set fire [to something]," the phrase 「火をつける」[hee-oh-tsu-keh-roo] is used ("Set Fire"). If you wish to say something "is fire" in the modern colloquial English equivalent of "that's cool," you can say 「ウケる」 [oo-keh-roo] for things that are funny or amusing, or simply 「いい」[like saying the letter "E"] to mean "that's good, that's cool."