The Burna Boyz are Orks in thrall to the sheer joy of fire, wielding burnas, part cutting-torch, part flamethrower, that gout searing promethium across anything unlucky enough to stand in front of them. They are drawn from the ranks of would-be Meks and grots-turned-tinkerers, and half of them are as interested in taking things apart with the cutting flame as in roasting the enemy, though on the battlefield the distinction rarely matters.
A mob of Burnas swarming into a trench or bunker is a nightmare of billowing flame and greasy smoke, flushing out cowering defenders and reducing them to ash before they can raise their guns. Interspersed among them, a few Spanner Boyz haul heavier kit, big shootas and rokkit launchas, adding proper dakka to the inferno.
The Burnas are unpredictable and more than a little unhinged, as likely to set fire to a fellow ork's backside for a laugh as to advance in good order. But when they reach the foe the result is always the same. Fire is honest, in the Orky way of thinking, and the Burna Boyz preach that gospel with every squeeze of the trigger.