What is the definition of Fard and Sunnah, what is the difference between them, and what is the ruling on one who abandons each of them?

"Fard" (Obligatory) is what the Sharia has commanded definitively; its doer is rewarded, and its ترك is sinful. As for "Sunnah," it is everything attributed to the Prophet, peace and blessings be upon him, whether it be a saying, an action, or an endorsement. Among the jurists, it is that which contrasts with the "Wajib" (necessary), and it includes the "Mustahabb" (recommended) and the "Mandūb" (desirable); its ترك is not sinful.