As an ocean wave nears the shore, the bottom part of the wave begins to slow down due to the drag of the bottom surface it is traveling over, but the top of the wave continues traveling at the same speed. Eventually the top of the wave will be traveling faster than the bottom of the wave. The wave then begins to change shape and the top of the wave crashes onto the shore.

http://theonlinelearningcenter.com/Assets/module_medias/Module37_wave1.mov