One blaze rod can smelt twelve items, burning for 120 seconds. One coal/charcoal can smelt eight items, burning for 80 seconds. Blaze rods are the best non-lava bucket source of smelting fuel in the game.

You'll need to hunt down a blaze, hope it drops a rod, and then crush that rod into powder on a crafting table if you want to acquire some. Once you've got some, there are several things you can craft with it.