It is a book in the writings section of Hebrew Scriptures -- called the Old Testament by Christians. It is particularly important to conservative Christians because of its apocalyptic content. From the book, they have derived beliefs concerning the end of the world and battle of Armageddon.
The book describes the life of Daniel, a Jewish prophet, who is described as having lived in the sixth century BCE. Many religious conservatives believe that it was actually written by Daniel. Many religious liberals believe that Daniel is probably a mythical character and that the anonymous author lived in the second century BCE.