What is the difference between a solitary (fard) hadith and a rare (gharib) hadith?

The terms 'Fard' (singular/individual) and 'Gharib' (rare/uncommon) are synonymous in technical usage.

An absolute Gharib is that whose rarity is in the origin of its chain of narration, such as the hadith, "Actions are but by intentions."

A relative Gharib is that which has multiple chains of narration initially, but a singularity (tafarrud) occurred with some narrators. This singularity might be relative to a specific person, such as the hadith, "I have been commanded to fight the people," or to a specific characteristic, such as the Prophet's (peace be upon him) recitation of Surat "Qaf" and "Al-Qamar" in the two Eid prayers.