newschool is rap with a high production value, lots of bass, mixes, etc (a lot of hard work goes into the music behind the rap). that's what Kanye does, that's what Nas has been doing lately (think Hero), that's what Jay-Z does.
Stuff like T-Pain and 50 on the other hand, is pretty basic and sounds like shit, hardly any work goes into their beats, therefore it isn't oldschool (good sound, low budget) or newschool (great sound, big budget), it's just a whole new class of shit.