Sunan Abu Dawud is a collection of sayings and deeds of Prophet Muhammad (SAW), also known as the sunnah. The reports of the Prophet’s sayings and deeds are called ahadith. Imam 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.
Imam 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 (SAW), however it is also known to contain some weak ahadith (some of which he pointed out, others which he did not).
It is important to realize, however, that Imam Abu Dawud’s collection is not complete: there are other scholars (i.e. Imam Bukhari, Muslim, Malik etc.) who worked as Imam Abu Dawud did and collected other reports.