The continental shelf is the portion of the seafloor that extends from the shoreline to the deep ocean.  In this area the ocean waters are relatively shallow, and the depth increases only gradually.  It can extend anywhere from a few yards to several miles.  Beyond it, ocean depth increases dramatically.  Scuba diving near it, it would look as if the ocean floor just dropped away, like an underwater cliff. 

The Continental shelf is also where most of the undersea life we know about is concentrated.  Most varieties of seaweed, coral, shellfish, and crustaceans live on Continental shelves.