Generally, the disheartening reality of a teenager coming to terms with how cruel the world is or seems to be to them, and attempts that follow to rationalize their disenchantment in a grossly asymmetric way. I.E., the overreaction of one towards a rejection of God or intrinsic moral value.
They most often find themselves manifesting a sort of philosophy most akin to an undeveloped form of existentialism, damn Sartre's rotting bones. Some actually begin to identify with the true nature of their convictions, instead of carrying on the deception of being a 'nihilist' just for sake of a cute, anti-cultural label.
The complete rejection of all value, all truth, all knowledge, &c., is beyond most people's strength of will or conviction. In short, few but the raving lunatics of society can actually be accurately described in this manner -- most that use the word, simply demand the attention it garners for them in their social circles.
Pragmatically, it's nothing but another puerile ploy for attention in the youth, that have not (yet?) found their independence of belief.
The American Underground Nihilist Society; @ anus.com -- an outstanding large number of pseudintellectual crackpots, screeching like banshees under the muse of nihilism.
Most gothic sorts could be described as pseudonihilism.