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.

Spawning. Blaze spawners can be found only in Nether fortresses and will start spawning blazes once a player gets within 16 blocks. The mob spawners have a chance of spawning 1-4 blazes at once. Blazes can only spawn in light level 11 or lower (at the lower half of their body).