What is the difference between Wajib and Fardh? Is the Sunnah after Maghrib Wajib, or Fardh, or Sunnah? What is the difference among these three?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

There is no difference according to the majority of the scholars between Fardh and Waajib. Fardh means Waajib and Waajib means Fardh (obligation).

However, the scholars of the Hanafi school of jurisprudence are of the view that there is a difference between Fardh and Waajib.

Fardh, according to them, is what is proved by absolute evidence (evidence that only has one meaning). And waajib is what is proved by presumptive evidence (Evidence that could have more than one meaning).

The Sunnah, according to the jurists is an act of worship for which the doer is rewarded, and the the person who does not do it is not punished.

The two Rak'ahs after Maghrib prayer is not Fardh or Waajib but a Sunnah, as the Prophet answered the Bedouin who asked him whether he has to perform prayers other than the obligatory ones, the prophet said:"Except that which you perform of your own free will."

However, it is desirable to perform the 2 Rak'ahs after Maghrib, as the Prophet said: "Whoever regularly  performs  12 voluntary Rak'ahs in 24 hours will enter Paradise: 4 Rak'ahs before Dhuhr, 2 Rak'ahs after Dhuhr, 2 Rak'ahs after Maghrib, 2 Rak'ahs after 'Ishaa, and 2 Rak'ahs before Fajr." This is an authentic Hadeeth reported by An-Nasaa'i .

All these prayers are called Rawaatib Sunnah, and a Muslim should perform them, but he is not sinful if he does not perform them.

Allaah Knows best.