What Is a Maya Codex?:
Codex refers to an old type of book made with pages bound together (as opposed to a scroll). Only 3 or 4 of these hand-painted hieroglyphic codices from the Post-classical Maya remain, thanks to environmental factors and zealous purging by 16th century clergy. The codices are long strips folded accordion-style, creating pages about 10x23 cm. They were probably made from the inner bark of fig trees coated with lime and then written on with ink and brushes. The text on them is short and needs more study. It appears to describe astronomy, almanacs, ceremonies, and prophecies.
- Maya Codices - From Archaeology @ About.com
Why Is It 3 or 4?:
There are three Maya Codices named for the places they are currently located, Madrid, Dresden, and Paris. The fourth, possibly a fake, is named for the place it was first shown, the Grolier Club of New York City. The Grolier Codex was discovered in Mexico in 1965, by Dr. José Saenz. In contrast, the Dresden Codex was acquired from a private individual in 1739.
Unfortunately, the Dresden Codex suffered (especially, water) damage during the Second World War. However, before then, copies were made that continue to be of use. Ernst Förstemann published photochromolithographic editions twice, in 1880 and 1892. You can download a copy of this as PDF from the FAMSI website.
The Madrid Codex:
The 56-page Madrid Codex, written front and back, was split in two pieces and kept separate until 1880, when Léon de Rosny realized they belonged together. The Madrid Codex is also called Tro-Cortesianus. It is now in the Museo de América, in Madrid, Spain. Brasseur de Bourbourg made a chromolithographic rendition of it. FAMSI provides a PDF of the Madrid codex.
The Paris Codex:
The Bibliothèque Impériale acquired the 22-page Paris Codex in 1832. Léon de Rosny is said to have "discovered" the Paris Codex in a corner of the Bibliothèque Nationale in Paris in 1859, after which the paris Codex made news. It is called the "Pérez Codex" and the "Maya-Tzental Codex", but the preferred names are the "Paris Codex" and "Codex Peresianus". A PDF showing photographs of the Paris Codex is also available courtesy of FAMSI.