Pseudepigrapha literally means false writing. It gets its name from the fact that the books that are classified as such have false author claims. Such a book is the Gospel of Thomas which is not accepted to have been written by Thomas but instead written several centuries after that. Another book is the Book of Enoch which claims to have been written by the Enoch of Genesis who “walked with God and then was no more.”
While numerous accepted books of the Bible have been attacked by liberal scholars as having been written much later – for example some claim the book of Daniel could not have been written until the 1st or 2nd century BC – books that are classified as pseudepigrapha have essentially no support for their authorship claims. They are almost universal understood not to have been written by the claimed author and almost never was even written during their lifetime.
See also: apocrypha, canon of scripture
The canon of scripture are the accepted books of the Bible. The Protestant church has 66 books, 39 in the Old Testament and 27 on the New Testament. The Catholic, Anglican, Ethiopian, and Greek Orthodox churches all have additional books that they consider scriptural. These books are considered apocryphal as they are not unanimously supported but instead questioned by different sects of Christianity.
While many factors decided what ultimately is considered scripture some of it simply comes down to tradition. The early church recognized from the beginning that certain books and letters carried important significance and thus circulated books as such. Paul recognized that he was writing scripture but certainly not all of his writings were scripture. His letters to the Corinthians reference additional letters that he wrote that are not a part of the Bible and have not survived.
By the 400’s AD the canon of scripture was relatively settled with the 27 accepted books of the New Testament. All Christian churches agree on these books although different churches have the books in a slightly different order. The apocryphal books are all in the Old Testament of each church’s Bible.