Rab Ne Bana Di Jodi and Jaane Tu... Ya Jaane Na fall in the AA category of box office success. This means that the films made more than double the amount of the money invested in it.
Rab Ne worked for many reasons -- its hero Shah Rukh Khan, director Aditya Chopra, and the fact that people probably wanted some entertainment after the horror of the Mumbai attacks.
On the other hand, Jaane Tu... worked because of its freshness -- the film starred newcomer Imran Khan and the not overexposed Genelia D'Souza. It was directed by debutant Abbas Tyrewala, who gave a fresh spin to romance. And then, there was the perfect marketting machine called Aamir Khan, the film's producer.
Also see: Showcasing Rab Ne Bana Di Jodi