Birth of a Wave
Wind creates most of the waves on the ocean. Friction between the air and the sea surface, transferring energy from wind to water, typically generates wave motion.
Heavy wind, as in a storm, creates a confused bed of superimposed waves called a "sea." The size of a wind-blown wave depends basically on how much energy the air transfers to the water; this is impacted by factors like wind velocity and distance.
Waves flow from the "generating area"--the wind source--in the same direction the air was blowing. Even after the winds die, the waves continue sloughing where they were directed and become more regular as they travel. These long-distance, stable waves are "swells."
Swells can travel great distances across the ocean. Because waves don't lose much energy traveling over deep water, they can move mostly unchanged unless countered by a stronger swell from another direction or until they hit the shore.
Other Swell Sources
While wind is the main generating force for swells, long-distance waves can also be prompted by seismic activity, volcanic eruption and tidal activity. These waves can sometimes be very big indeed, especially when they encounter shallow waters. Tsunamis, or seismic sea waves, are not particularly noticeable as swells in the open ocean--they have a low crest and a long wavelength--but can sometimes approach 100 feet in height when hitting a coastline.