Short answers, pulled from the story.
The Histories covers Roman history from AD 69, following the death of Nero, through to AD 96, the end of Domitian's reign. The surviving portion of the work reaches only the year 70 and the very beginning of Vespasian's reign.
The first four books of the Histories survive in full, along with the first twenty-six chapters of the fifth book. Together with the Annals, the Histories originally totalled thirty books, of which roughly half have survived; tradition assigns fourteen of those books to the Histories.
The four emperors were Galba, Otho, Vitellius, and Vespasian, each of whom took power in quick succession during AD 69. Otho defeated Galba, Vitellius defeated Otho at the Battle of Bedriacum, and Vespasian's troops then murdered Vitellius.
Tacitus wrote the Histories approximately thirty years after the events of AD 69, with the work composed around AD 100-110, not long after Nerva's seizure of power.
Book 5 of the Histories contains a short ethnographic survey of the ancient Jews, inserted as a prelude to the account of Titus's campaign in the First Jewish-Roman War. Scholars regard it as an invaluable record of how educated Romans understood that people.
Tacitus argues that only the principatus, the monarchical emperor, could maintain the peace and cohesion of the empire. He believed the ideal ruler should be neither a tyrant like Domitian nor a political incompetent like Galba, and he considered the adoptive emperors the only realistic solution to the empire's structural problems.