Blaze are floating mobs only found in the nether. They're essential because they are the only source for Blaze Rods. The second a Blaze sees you, it'll begin attacking you and will shoot three fireballs towards you. Listed below is how much damage a blaze can do depending on what difficulty you're playing on.

This gives a blaze 20 health points (because 1 heart = 2 health points). To kill a blaze, you need to inflict 20 points of damage to the blaze.