Sunan Abu-Dawud is a collection of sayings and deeds of Prophet Muhammad (peace and blessings be upon him) (also known as the sunnah). The reports of the Prophet's sayings and deeds are called ahadith. Abu-Dawud lived a couple of centuries after the Prophet's death and worked extremely hard to collect his ahadith. Each report in his collection was checked for compatibility with the Qur'an, and the veracity of the chain of reporters had to be painstakingly established. Abu-Dawud's collection is recognized by the overwhelming majority of the Muslim world to be one of the most authentic collections of the Sunnah of the Prophet (peace and blessings be upon him), however it is also known to contain some weak ahadith (some of which he pointed out, others which he did not).
Abu-Dawud (full name Abu-Dawud Sulaiman bin Al-Aash'ath Al-Azdi as-Sijistani) was born in 202 A.H. and died in 275 A.H. He was one of the most widely travelled of the scholars of ahadith, going to Saudi Arabia, Iraq, Khurasahn, Egypt, Syria, Nishapur, Marv, and other places for the sole purpose of collecting ahadith. His primary interest was in law, hence his collection focuses purely on legal ahadith. From about 50,000 ahadith, he chose 4,800 for inclusion in his work based on their superior authenticity.