A revealed religion is one based on information communicated from the spiritual world to humanity through some sort of medium, most commonly through prophets. Thus, spiritual truth is revealed to believers because it is not something inherently obvious or something one could naturally conclude.
Judeo-Christian Religions as Revealed ReligionsThe Judeo-Christian religions are all strongly revealed religions. The Old Testament includes many stories of those whom God used to transmit knowledge of himself and his expectations. Their appearance comes at times when the Jewish people have significantly strayed from God's teachings, and the prophets remind them of his commandments and warn them of impending disaster as punishment. For Christian, Jesus arrived as God incarnate to directly minister to the community. For Muslims, Mohammad was selected after Jesus (seen as a prophet rather than as God) to provide a final revelation.
Writings about these prophets exist today which continues to guide believers. The Tanakh, the Bible, and the Koran are the scriptures of these three religions, providing the most basic building blocks of their respective faiths.
More recent religions drawing on Judeo-Christian teachings are also generally revealed religions. The Baha'i faith accepts that God chose prophets all over the world to reveal his messages, and those prophets have continued passed the time of Mohammad. Raelians accept the Judeo-Christian prophets as those who communicated with aliens rather than God, and their founder, Rael, as being the most recent prophet of the alien Elohim. Knowledge of the Elohim comes only from Rael, as they do not directly communicate with anyone else. As such, Raelianism is every bit as much a revealed religion as its more traditional predecessors.
Natural ReligionThe opposite of revealed religion is sometimes called natural religion. Natural religion is religious thought that is independent of revelation. Taoism is an example of natural religion, as are all forms of Satanism, among others. These religions have no divinely inspired books nor prophets.
"Man-Made Religion"The term "revealed religion" is sometimes pejoratively used synonymously with "man-made religion," implying that these religions tell people what other people claim to know about God rather than people learning about God directly through study and experience.
Deists are fairly vocal in this regard. They believe in a creator that is knowable through his creation but disregard the idea of any authority on the matter, particularly when they claim unprovable things. They do not necessarily deny supernatural events, but they do not accept them as fact except perhaps through personal, subjective experience. The stories of others are not considered a valid basis for one's own understanding of God.
Necessity of RevelationOf course, those who believe in revealed religion find an absolute necessity in revelation. If a god or God does indeed have expectations for humanity, those expectations need to be somehow communicated, and traditionally information has spread via word of mouth. So God reveals himself through prophets who pass on the information to others who eventually write such information down so that it can be further shared. There is no objective measurement of the value of revelation. It is a matter of faith whether you accept such revelations as genuine.