Seas are created when the wind blows on the surface of the water, creating movement. With enough wind over a large enough area for a long enough time, swell forms, which become organised, well spaced patterns.

Picture what happens when you drop a rock into a still pond. A swell is generated.

The Swell then travels until worn out by time, distance or obstacles. Generally, "Swell" refers to open ocean waves, "Waves" are what people want to go stick their surfboard on.