Ocean waves are an expression of energy moving through water. The most common ocean waves are caused by surface winds pushing water toward land. Seismic activity can also cause waves, such as tsunamis created by undersea earthquakes or landslides. Undersea currents can also influence wave formation, creating small oscillations deep in the ocean that can intensify once the waves approach a coastline.
In the open ocean, waves are called swells. These are responsible for the up-and-down motion of boats on the sea, and they are caused by leftover kinetic energy perpetuating through the seawater. As waves reach shallower water, however, the rising ocean floor constricts the lower portion of the waveform, causing the upper reaches to amplify. Eventually, the wave becomes too tall to support itself, and it collapses or crashes. This is also why a tsunami may start out just a meter or two high in the deep ocean and rise to life-threatening levels once it hits the shoreline.
High and low tide are actually waves, created by the interaction of the gravitational fields of the moon and sun. The pull of these celestial bodies creates bulges in the ocean, and as the Earth rotates, the bulges create tides by effectively raising and lowering the water level.