Gaius (2nd century AD); Ulpian (AD 170-223/4); Paulus (fl. c. AD 200), are particularly significant because the texts survived independently of the Justinian compilations, and therefore represent earlier ideas: Gaius’ Institutes; Paulus’ Sententiae; Ulpian’s Regulae.
For Gaius, Ulpian and Paulus see FIRA, Girard and Collectio t.1 and t. 2.
Fragmenta Vaticana, which includes both imperial enactments and abstracts from the writings of Papinian, Ulpian and Paulus are reproduced in Collectio t.3, FIRA, Girard and Huschke.
Justinian’s works are known collectively as the Corpus Juris Civilis (body of civil law) and consist of: the Digest (or Pandects), Institutes, Code and Novels.
The Digest, Institutes and Code are restatements or revisions of existing law. The Novels are Justinian’s subsequent enactments.
About 892 AD during the reign of Leo VI, Eastern Roman Emperor, a Greek abridgment of Justinian's laws was promulgated: Τà Βασιλικα