Understanding Fard and Sunnat & Haraam and Makrooh (In Brief)
Fard, Waajib and Sunnat are terms used in the realm of Fiqh. For understanding purposes in practical living they are all regarded as Waajib or compulsory.The difference lies not in practice, but rather in consequences. eg. If you leave out a Fard there is Qaza but there is no Qaza for Sunnats (only sincere taubah). In Salaat leaving out a Fard invalidates Salaat, Waajib requires Sajda e Sahw and ommiting Sunnat is a sin but if one genuinely forgets or has a pressing need in an odd occasion then it does not invalidate the Salaat. Similar is the case of Haraam and Makrooh. In fact, many Kitabs use the word Makroh inplace of haraam and vice versa and waajib for Sunnat and Fard. Imaam Ahmad ibn Hambal Rahmatullahi Alay was once asked as to what are the Waajib acts of Salaat. He replied that ALL the acts of Salaat are Waajib but some Waajib acts require Sajda e Sahw.