It can be a bit of a contentious issue, but this is a description which I found which is fairly balanced and non-emotionally charged(!):
The Apocrypha refer to texts which are left out of officially sanctioned versions ('canon') of the Bible. The term means 'things hidden away,' which implies secret or esoteric literature. However, none of these texts were ever considered secret.
In some Protestant Bibles, they are placed between the New and Old Testament. In the Roman Catholic Bibles the books are interspersed with the rest of the text. In this case they are also called 'Deuterocanonical', which means 'secondary canon.'
Jerome rejected the Deuterocanonical books when he was translating the Bible into Latin circa 450 CE. This was because no Hebrew version of these texts could be found, even though they were present in the Greek Old Testament (the Septuagint). However, they eventually were accepted by the Church, and most of them remained part of the Bible. Protestants rejected these books during the Reformation as lacking divine authority. They either excised them completely or placed them in a third section of the Bible. The Roman Catholic Council of Trent, on the other hand, declared in 1546 that the Deuterocanonical books were indeed divine.
Of these books, Tobias, Judith, the Wisdom of Solomon, Baruch, and Maccabees, remain in the Catholic Bible. First Esdras, Second Esdras, Epistle of Jeremiah, Susanna, Bel and the Dragon, Prayer of Manasseh, Prayer of Azariah, and Laodiceans are not today considered part of the Catholic apocrypha.
With one exception, all of these books are considered 'Old Testament'. The apocryphal New Testament 'Letter of Paul to the Laodiceans', was once incorporated in many versions of the Bible. However Laodiceans is now considered just a pastiche of other Epistles, and is omitted from contemporary Bibles.
There are many other apocryphal books, which do not fall into the 'Deuterocanonical' category, such as the many additional New Testament Gospels, and the apocalyptic book of Enoch. Some of these can be found in the Apocrypha section.