the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
With respect to the field of Islamic jurisprudence, most scholars use the two terms synonymously. They are defined as follows:
Fard: Obligatory, failure to perform it is sinful.
Wâjib: Obligatory, failure to perform it is sinful.
These words are synonymous to most scholars of Islamic jurisprudence. However, Hanafî scholars differentiate between the two. Fard refers to obligations that are more serious and the one who neglects them incurs a greater sin. One who rejects their obligatory nature is considered an unbeliever. Neglecting something wâjib is sinful as well, but not to the same degree. One who contests their obligatory nature remains within the fold of Islam.
The reason for the distinction is the strength of the evidence that exists to establish the obligatory nature of the matter in question.
For example, the Witr prayer, according to the Hanafî school of thought, is wâjib, but it is not fard.
Source: Islam Today