Short answers, pulled from the story.
The Historia Augusta is a late Roman collection of biographies, written in Latin, covering the Roman emperors, their junior colleagues, designated heirs, and usurpers from 117 to 284. It presents itself as the work of six authors called the Scriptores Historiae Augustae, writing during the reigns of Diocletian and Constantine I, but scholars now generally believe it was composed by a single anonymous author in the late 4th or early 5th century.
The text names six authors collectively called the Scriptores Historiae Augustae: Aelius Spartianus, Julius Capitolinus, Vulcacius Gallicanus, Aelius Lampridius, Trebellius Pollio, and Flavius Vopiscus of Syracuse. Hermann Dessau argued in 1889 that all six were fictitious, and by the early 21st century the consensus among scholars supported the theory of a single unknown author writing after AD 395, though the debate has never been fully resolved.
The text claims to have been written during the reigns of Diocletian and Constantine I, roughly in the late 3rd and early 4th century. Most scholars now believe it was composed in the late 4th century or early 5th century, with Ronald Syme dating the composition closely to around AD 395. Alternative proposals range from the reign of Constantius II to somewhere between 361 and the 380s.
Scholars treat it with extreme caution. The work contains around 150 allegedly authentic documents, including 68 letters and 60 speeches, nearly all of which have been rejected as fabrications. Estimates for some biographies put the proportion of reliable historical detail as low as 4 or 5 percent. Its value lies partly in the fact that it is the only continuous Latin account for much of its period, and it preserves some details confirmed by inscriptions, such as the construction of Hadrian's Wall during Hadrian's reign.
The oldest surviving manuscript is Vatican Pal. lat. 899, known as the Codex Palatinus, written at Lorsch in the first quarter of the 9th century. It carries six centuries of editorial corrections from figures including Petrarch and Poggio Bracciolini. The editio princeps was published in Milan in 1475, based on a copy of the Codex Palatinus thought to have been made for Petrarch in 1356.
Isaac Casaubon coined the title Historia Augusta in his critical edition of 1603. The title as recorded on the Codex Palatinus manuscript is Vitae Diversorum Principum et Tyrannorum a Divo Hadriano usque ad Numerianum Diversis compositae, meaning The Lives of Various Emperors and Tyrants from the Divine Hadrian to Numerian by Various Authors.