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Threats

Natural threats such as hurricanes, tsunami's (tidal waves) or various diseases do damage coral reefs, but the reefs generally are able to recover after such events. However, just as a a stressed person is more likely to get sick or have a harder time recovering from an injury,  added stresses from human actions weaken the corals so that they may not recover from natural events. 

Global Warming - Funny as it sounds there's nothing to laugh about ...  Global warming is a deadly event, particularly for corals.  A rise of two degrees in water temperatures stresses them out and they expel the algae living within them. This deprives the coral of their color, bleaching them white, and of essential nutrients the algae provides. Coral can survive minor bleaching from one stress such as warmer water, but add together many minor stresses at one time, and the coral will likely die. 

Sedimentation - Costal development of areas near coral reefs create more rain water wash off into rivers which then carry tons of sediment and fresh water into the coral reefs. The fresh water dilutes the salinity and sediment cuts off sunlight essential to zooxanthellae photosynthesis.  In tropical areas, mangrove trees and see grass beds growing near coral reefs have traditionally trapped sediments before they reach the reef.  Now though, in areas like Fiji, logging in the rainforests and destruction of mangroves has produced excess sediment flowing down river and smothering the coral reefs.

Coke Dynamite- a one liter coke bottle found off of Capone islands contains sediments of explosives.

Over Fishing- Over fishing is a major problem for coral reefs. If all the kelp eating fish are taken away, the faster growing kelp easily overruns the coral reefs, shadowing and killing the coral.  Also, as fishers get more and more desperate for fish, they use much more destructive methods of gaining them. Some people have resorted to dynamite (blowing dead fish to the surface) and cyanide (capturing "stunned" fish while their slow from the poison). Both these tactics kill any surrounding organisms as well as the coral, so that any surviving animals no longer have a home.

Pollution - Toxic chemicals are just as harmful to coral as they are to humans or any other living being.  Sewage dumped into the ocean adds nutrients that allows kelp to thrive and upsets the balance between slow growing coral and fast growing kelp.  Algae also thrives on the added nutrients and blankets the water, cutting off the sunlight that the coral's zooxanthellae algae needs to produce food.

Recreation- Careless people boating and collecting coral around coral reefs can easily damage the reef. 

To help conserve coral reefs people could conserve and recycle, so waste doesn't end up in the ocean and participate in actions that help reduce global warming.  Volunteer,  or make donations to coral reefs or ask your local aquarium what they are doing and how your donation helps. Be informed about products you buy especially animals for your aquarium, and generally educate yourself about laws or measures that affect the reefs.  For more ideas a simple search on the internet will grant you thousands and you can check out this website - http://www.publicaffairs.noaa.gov/25list.html