The entire Bible would have been a roll of immense length. It would have been clumsy and impractical. It would have made the location of a text very difficult, especially if the desired text occurred toward the middle or end of the roll. Hence we see the emergence, in the fourth century of our era, of the "codex" or book in our sense of the term. The "codex" or book was possibly a Christian invention and was perhaps introduced for the first time in the Christian Bibles. It not only made easier the location of a particular text but put together in a single volume all the books of the Old and New Testaments.
The oldest existing codices of the Christian Bible are all parchment copies, written in uncial letters, and dating from the fourth century. Among these oldest existing Bibles the following are the more important:
1. The Codex Vaticanus, dating from the first half of the fourth century and preserved in the Vatican Library. It represents a form of text current in Egypt in the second century.
2. The Codex Sinaiticus, also dating from the fourth century and representing the same form of text as the preceding. It was discovered in 1844 in the Monastery of St. Catherine on Mt. Sinai, and is now kept in the British Museum.
3. The Codex Alexandrinus, belonging to the fifth century. It was brought from Alexandria to Constantinople and later transferred to the British Museum in London.
4. The Codex Ephraemi, also belonging to the fifth century. It is a palimpsest: Some writings of St. Ephraem were written across the Biblical text, which had been more or less erased but is still legible.
The naming of these Bibles is largely accidental. One is designated by its place of origin (Alexandrinus), another by its place of custody (Vaticanus), another by its place of discovery (Sinaiticus), and the last by the special character of its manuscript (Ephraemi).
Material taken from http://www.cathtruth.com/catholicbible/bookbook.htm