the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
The various types of prayer are as follows:
Fard: Obligatory, failure to perform it is sinful.
Wâjib: Obligatory, failure to perform it is sinful.
For prayers, these two terms are synonymous to most scholars. Hanafî scholars differentiate between the two. Fard prayers are more serious and the one who neglects them incurs a greater sin. Neglecting a wâjib prayer is sinful, but not to the same degree.
The reason for this distinction is the strength of the evidence that exists to establish the obligatory nature of the prayer. The Witr prayer, according to the Hanafî school, is wâjib, but it is not fard.
Sunnah Mu’akkadah: Emphatic practice of the Prophet (peace be upon him); it is strongly recommended to perform this prayer. Failure to do so on occasion is not sinful, according to the opinion of most scholars.
Sunnah: Non-emphatic practice of the Prophet (peace be upon him); performing this prayer is highly rewarded but failing to do so is not sinful.
Nafl: Any purely voluntary prayer.
Source: Islam Today