As you'd probably already know, all Orks have a modicum of psychic energy. This is what keeps all of their 'machinery' running, and if they were so inclined they could even think that they'd conquer the galaxy and actually succeed. What Weirdboyz do is crank that psychic energy up to eleven. The Orky equivalent to Librarians or other races' psykers, Weirdboyz are Orks who have fully mastered their psychic abilities, and are unique in that they absorb the psychic energy from other Orks, therefore empowering their abilities. If they cannot release excess energy in time, though, the Weirdboy'll get 'Eadbanged (its head explodes). Sometimes, Weirdboyz are purposely lured to enemy lines just to detonate as living suicide bombs, whereupon they become dead suicide bombs, of course.
On the tabletop, their save may as well not exist, and their Leadership is poor enough that a Perils of the Warp can cause them some serious trouble. Combined with their short ranged and combat-based powers, Weirdboyz are very reckless compared to other psykers, and you can't count on them to survive an encounter with most units. They also can't call a WAAAGH! anymore, which is a disappointing change that has stopped a few fun (if gimmicky) lists. Weirdboyz are not amazing psyker material, and they in no way make Orks capable of the kind of Warp Charge factory that Daemons can do.
So why go through the trouble of taking them? Well, they got a big improvement in that their powers are as random as those of any other psyker, instead of being rolled for each turn. They generate Warp Charge properly, use their powers like everyone else, and they even got a force stave into the bargain. Weirdboyz also generate an extra Warp Charge if there are 10 or more Orks within 12″, which is pretty easy to do. This rule does have a small downside where the Weirdboy might take damage if he doesn't pass a psychic test in the same turn, but he only suffers a Wound on a 6, so it's not too dangerous.
In conclusion, they're here to blow shit up with their mind-powers, and not much else.
Warpheadz, unlike their normal Weirdboy counterparts, actually enjoy the painful sensation of absorbing psychic energy, and thus can control their powers even better than their contemporaries.