What is the difference between sunnah, fardh and wajib? And in which case "witr" lie? An early response would be highly appreciated.
The terms of Fardh and Sunnah in this context are not the terms of the basic religious sources. These were not used even in the traditions ascribed to the Holy Prophet (sws). Basically the prayer we are obliged to offer regularly is the one we call Fardh. Sunnah in this context is used by some of the juristic schools like that of Imam Abu Hanifa, to connote Raka`ahs of the payer that are not mandatory, yet were performed or were recommended by the Prophet (pbuh).The Holy Prophet (sws) would offer nafal (supererogatory) prayer before or after the Fardh (obligatory prayers). Jurists of this school hold that the fact that the Prophet offered these prayers almost regularly raises the act from the merely being nafal (supererogatory). It gains some importance and should not be left out without reason.
I would reiterate the point that this distinction is not of much importance and should only serve to distinguish the obligatory from the supererogatory. Therefore, in my opinion, leaving a supererogatory on purpose should not be held punishable in any case.
Witr is optional for us. It was obligatory for the Prophet (sws). It is part of the tahajjud prayer.