Fringing reef off the coast of Eilat, Israel.
A coral reef is a large underwater wall made of dead and living corals. The reefs are formed in tropical marine areas(30 degrees north and south of the equater). It acts as the home of many tropical fish and other animals.
People look at them while snorkeling and diving. One example of a coral reef is in Malaysia at Pulau Tioman, off the State of Pahang.
Coral reefs can be very dangerous to ships, because they are usually underwater, but close to the top. This makes it hard for ships to see them, but are easy for ships to hit them.
Coral reefs are in danger of becoming damaged due to tourism. Coral reefs provide natual breakwater for the waves and most coral reefs are under threat of over fishing. Some of the most beautiful coral reefs are in Belize and one coral reef is 300km long!
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For more information (background, pictures, experiments and references): Coral Reefs