A sea is a large area of open water that is normally shallow. By contrast, an ocean is bigger and has much deeper water.

Many seas are areas of shallow water on the edges of land. They lie between land and oceans. The North Sea is an example, being surrounded with countries of Europe, but being on the edge of the Atlantic Ocean. During the Ice Age, the North Sea was dry land, so that shows how shallow it is.

Just off many continents there is an area of quite shallow water called the continental shelf. This is where most seas are. The Yellow Sea (between China and Korea) and South China Sea (between Indonesia and Malaysia and Vietnam) are examples of this. They are connected to the Pacific Ocean.