Sahih Bukhari is a collection of sayings and deeds of Prophet Muhammad (pbuh), also known as the sunnah. The reports of the Prophet's sayings and deeds are called ahadith. Bukhari 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. Bukhari'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 (pbuh).
Bukhari (full name Abu Abdullah Muhammad bin Ismail bin Ibrahim bin al-Mughira al-Ja'fai) was born in 194 A.H. and died in 256 A.H. His collection of hadith is considered second to none. He spent sixteen years compiling it, and ended up with 2,602 hadith (9,082 with repetition). His criteria for acceptance into the collection were amongst the most stringent of all the scholars of ahadith.
It is important to realize, however, that Bukhari's collection is not complete: there are other scholars who worked as Bukhari did and collected other authentic reports.