The Orks answer to Assault Marines, Stormboyz are atypical greenskins with even less instinct for self preservation, strapped into a highly unstable and barely functioning missile that we like to pretend is a Jump pack.
Stormboyz are straight up odd. When the orky lifestyle means you can do whatever you want whenever you want, not every young and bold ork actually does so; they're either overwhelmed by the possibilities like the average Garry's Mod player after five minutes, or simply too ornery from being a teenager and want to be contrarian. Either way they join the Stormboyz. The Stormboyz have their own sort of culture that's quite unlike most other greenskins. They respect authority, keep their uniforms clean... hell, they have uniforms. Stormboys don't just practice fighting by getting into a scrap like other orks, they actually train, and in battle this training seems to work. Stormboys are known for cunning plans and suspiciously well orchestrated (ork-estrated?) strikes. The 4th Ed Ork Codex entry on Stormboyz is renowned for being the most hilarious Codex entry in a book full of hilarious Codex entries.
The stormboyz are fast, faster than normal jump infantry. Not only can they cross impassible terrain with their packs, but they can choose to run for an extra 2d6" (risking some wounds if they do). If they don't take their normal move, they can get one reroll on their charge dice and add Hammer of Wrath to the attack. Finally, in a Waaagh! they can move 12" + 4d6 inches (with a single die reroll) to assault potentially up to 36", a feat that can be matched only by space wolf wulfen.
In statblock, Stormboyz are absolutely the same as typical boys at a slightly higher cost (+3 points). Given the benefits these silly rokkit packs offer, that's pretty much a steal, though depending on your situation a squad of shootas might do you better. Like standard boyz mobs, the Stormboyz can bring a single Nob along for a bosspole and powerklaw, and also like the boyz they can be taken in squads up to thirty. Nothing quite like thirty-one hooligans hopping 36 goddamn inches across the board to put some sharp bits into some heads.
There's also a popular git immortalised on suptg's archive, going by Uzgob Rokkit-Ranga, who WAAAGH'd where no Ork had WAAAGH'd before, nor since.
The pyro from TF2 as of the jungle inferno update is a good example of a burna-stormboy. OI WHAT'S A HUMIE DOIN' IN 'ERE! 'E'S NOT A GIT! *DAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKADAKKA*