Revelation, as a theological term, means to reveal through communication, messages from the deity. Almost all major world religions have some text that they hold to have been revealed from God. The Bible, for example, has as its last book perhaps the most famous one, the Book of Revelations, but it also several other forms of revelation through the Old Testament as well.
The Torah, or the Pentateuch (the first five books of the Bible), is traditionally held to have been given to Moses on Mt. Sinai, along with the Ten Commandments. Therefore, the various laws given to Moses can be considered to be the first of the Biblical revelations. Later, the books of the Prophets serve as different revelations, including warnings about the destruction of Jerusalem and the coming of the Messiah.
In the Christian tradition, the most famous revelation of the Bible is that of St. John, the last book of the New Testament. Also known as the Apocalypse, the author describes several visions regarding the end of the world and the Second Coming of Christ. There are, of course, several major interpretations of the Book of Revelations.
One major interpretation is that the various visions are referring to the contemporary situation of the author and the political situation in Rome. A second interpretation holds that the book foretells the end of the world, while another interpretation sees the work as an allegory regarding the daily struggle between good and evil.