Fard: obligatory prayer
Sunnah: Prophet's tradition
Wajib: obligatory prayer (Denying a Fard makes one a non-Muslim but denying a Wajib makes one a sinner. One must make up any missed Fard or Wajib prayers.)
Nafl: voluntary prayer
And again how sunnah is divided:
The sunnah are again divided into Sunnah Muakkadah and Sunnah Ghair Muakkadah.
Sunnah Muakkadah: means an act that Prophet Muhammad, Sall-Allahu alayhi wa sallam, always used to do. These rakats must be offered. (However if missed, there is no makeup or qada for it).
Sunnah Ghair Muakkadah: are sunnahs that Prophet Muhammad, Sall-Allahu alayhi wa sallam, used to leave sometimes without any reason. These are not obligatory.
Thanks for reading.
I felt many of us have lack of clarity on this basic concept of Namaz we offer.
Jazakallah hu khair.
Source: Received as whatsaap message. Republished in this blog.