Waves on the surface of the sea or a lake are caused by wind blowing over the surface. The longer the wind has been blowing in the same direction and the deeper the water is, the larger the waves can get. Waves usually only break and make the familiar frothy edge on the wave when they reach the shore. They sometimes break out at sea when they get too steep. Surfers like to surf on breaking waves. The wave in the picture is breaking onto a beach.