Daniel purports to be the account of a high ranking Jewish official in the court of Nebuchadnezzar. The first half is largely heroic stories where the protagonists are protected from certain death by adhering strictly to Jewish law (e.g. not bowing to false gods). The second half is a series of increasing detailed prophecies about the coming 4 or 5 hundred years of history.
In reality the book is more likely a pious fraud written during the Maccabean revolt in the 2nd century BC. Besides the obvious fact that it's natural to be skeptical of claims of prophecy the following reasons are usually given to support the claim that this is not authentic prophecy.
- The history of Babylonia reported in Daniel doesn't match up with extra-biblical cources (E.g. lists wrong king of Judah when Nebuchadnezzar brings Jews into exile)
- The predictions between 6th century BC and 2nd century BC don't match well with the modern understanding of the history of that time period. (Believed that the Medes were more powerful than Persians, whereas they had been conquered by the Persians)
- The predictions about the early part of Maccabean revolt against Antiochus IV are suddenly very accurate (Battles, forced Hellenization ofr Jews, etc)
- The later predictions about the Maccabean revolt are suddenly not accurate again. (E.g. the immanent end of the world didn't occur, Antiochus didn't die where predicted)
Daniel is a favorite of evagelical end times theorists since it's often vivid imagery can be matched Rorschach like to just about any current event.