Short answers, pulled from the story.
The Praetorian Guard's origins trace to bodyguards for Roman generals during the Republic, with the first historical record dating to around 275 BC and the Scipio family. Augustus formally established the Guard as the imperial bodyguard after 27 BC, appointing the first two Praetorian prefects, Quintus Ostorius Scapula and Publius Salvius Aper, in 2 BC.
The Praetorian Guard held unique legal rights, most notably the exclusive permission to bear arms inside Rome's sacred boundary, the Pomerium. Combined with their direct access to the emperor, their role in succession, and the donativum cash gifts each new emperor paid to secure their loyalty, they became capable of assassinating rulers and proclaiming replacements, as they demonstrated repeatedly from the assassination of Caligula in AD 41 through the auction of the throne in 193.
Under a system called sesquiplex stipendum, the Praetorian Guard received pay-and-a-half compared to legionaries. If a legionary received 250 denarii, a guardsman received 375 per year. Under Nero the pay was three and a half times that of a legionary. Domitian and Septimius Severus later raised the annual payment to 1,500 denarii, distributed in January, May, and September.
Constantine the Great disbanded the Praetorian Guard in AD 312 after defeating them at the Battle of the Milvian Bridge, where they had supported the rival emperor Maxentius. Constantine scattered the surviving soldiers to distant parts of the empire and demolished the Castra Praetoria, their barracks in Rome.
In AD 68-69, the Praetorian Guard was central to every shift of power. The prefect Nymphidius Sabinus turned the Guard against Nero in favor of Galba, who then refused to pay the promised 7,500 denarii per man. Otho bribed 23 speculatores to proclaim him emperor, leading to the lynching of Galba and his designated successor Piso. After Vitellius defeated Otho, he disbanded the Guard and replaced it with sixteen cohorts of his own loyal troops.
Originally the Guard recruited almost exclusively from central Italy, specifically Etruria, Umbria, and Latium according to Tacitus, with recruits aged 15 to 32. A candidate needed good physical condition, good moral character, a respectable family background, and letters of recommendation from prominent Roman figures. Septimius Severus changed this after 193, rebuilding the Guard from battle-hardened soldiers of the Danube legions.