The isnad of a hadeeth is its chain of narrators, from the person who recorded it (e.g. Imam Bukhari) to the person he heard it from to the person they heard it from and so on, all the way back to the Prophet (saws). Ahadeeth are primarily classified based on the continuity of their isnad and the reliability of the reporters that comprise it.

Chain of transmitters, the list of people who successively narrated a given hadith

"Chain of authority," the chain of people who conveyed a hadith from the Prophet (PBUH). Scholars who specialized in hadith, such as al-Bukhari and Muslim, subjected the isnad of each hadith they came across to intense scrutiny. Only if they were certain that each person in the isnad was competent and truthful, and that each had been in a position to meet and learn from or teach the next person in the chain, would the scholars accept a hadith as being authentic.