Short answers, pulled from the story.
The Year of the Four Emperors was AD 69, the first civil war of the Roman Empire, during which Galba, Otho, Vitellius, and Vespasian each ruled in succession. It marked the end of the Julio-Claudian dynasty and the beginning of the Flavian dynasty. The turmoil began with revolts against Nero in 68 and was not fully resolved until late 69 and into 70.
Nero took his own life on the 9th of June 68 after the Senate declared him a public enemy. The Praetorian Guard had abandoned him following the persuasion of their deputy prefect, Nymphidius Sabinus, who promised each guardsman 30,000 sesterces in Galba's name. With no military support and facing capture, Nero died by suicide.
Otho became emperor on the 15th of January 69 after bribing the Praetorian Guard to murder Galba in the Forum. His reign lasted a little more than three months. After his forces lost the First Battle of Bedriacum on the 14th of April 69, he took his own life rather than continue the civil war.
Vespasian was acclaimed emperor in Alexandria on the 1st of July 69, securing Egypt's vital grain supply. The Danubian legions declared for him in August and, led by Marcus Antonius Primus, defeated Vitellius's forces at the Second Battle of Bedriacum in October. Vitellius was killed on the 20th of December 69, and the Senate recognized Vespasian the following day.
The Batavians were a Germanic provincial people who, under a leader named Civilis, fought the Rhine legions beginning during Vitellius's acclamation. They aided the Flavian cause against Roman legions loyal to Vitellius. Some scholars argue that Flavian writers, including Tacitus, later recast this alliance as a Batavian revolt to conceal the embarrassing fact that the new regime had used provincial fighters against Roman soldiers.
Vespasian founded the Flavian dynasty, which succeeded the Julio-Claudians. The Flavians ruled from AD 69 to AD 96. Vespasian himself died of natural causes in 79.