The codex format was probably developed as a way to make the contents of a document more readily accessible in contrast to the roll. Callimachus, a cataloguer of books at the great library of Alexandria, is noted to have said, "A big roll is a big nuisance."
As to the relative frequency of use of the roll and of the codex, in an enumeration of 476 second-century non-Christian literary papyrus manuscripts from Egypt, 465 or more than 97 percent are in the form of the roll; but eight Christian biblical papyri known from the same century are all in the form of the codex. Likewise in the entire period extending to shortly after the end of the fourth century, out of 111 biblical manuscripts or fragments from Egypt, 99 are codices. That the codex increased in use in comparison with the use of the roll is natural in view of the many obvious advantages of the leaf book, not the least of which is that it is more feasible to write on both sides of a leaf, and hence such a book is cheaper. But the statistics just given indicate a particular and very early preference for the codex form on the part of Christians. This also is natural in view of the advantages of the codex with respect to matters of particular interest to the Christians. For example, the single Gospel according to Luke would probably have filled an average papyrus roll of approximately thirty feet in length, and Paul's ten collected church letters (including Philemon) would probably have occupied two ordinary rolls, but all Four Gospels or all of the Letters of Paul could readily be brought together in a single codex book. Likewise it is much more difficult to turn quickly to a specific passage in a roll, and much easier to do so in a codex.
Finegan, Encountering New Testament Manuscripts, p. 29.
Papyrus codices are the first to appear and are then joined by codices made of parchment. Almost all of the manuscripts of the New Testament are in the form of the codex. In fact, some believe the codex had its origins in early Christianity and was adopted by others later on.
The most common form of the codex was made up of a number of "gatherings" sewn together and placed within a sturdy covering. Each "gathering" starts out as four sheets which are folded in half to make eight leaves (folios) (16 pages). This "gathering" was called a "quire."
One of the languages into which the New Testament was first translated is Coptic. Here are two images (in color) available from the Copt-Net.
St. Mark 
St. Luke 