The sea concept comes from the latin mother and means the mass of salt water covering a large part of the Earth's surface, shown in blue on the globe and world map. Also called sea to each of the parties where we consider that this water mass is divided (the Mediterranean Sea, Aegean Sea, etc.).
In addition, a sea is a Lake of great extension. In these cases, told then closed or inland sea: the dead sea is one of the Lakes.
In comparison, we can say that the sea is an extension of salt water which the extension is less than that of the ocean. However, some do not distinguish between the sea of the ocean, or even the specialists.
Among the many classifications of the seas, we will mention the nearshore or coastal seas (which are large and fairly open, such as the sea of Norway), inland seas (they are found within continents and communicate with the oceans through a shallow Strait. For example: the Mediterranean Sea) and internal or closed seas (such Lakes of great extent occupying endorheic depressions).
Finally, it is noted that the concept of sea is used to refer (in exaggerated way) to the high abundance of things figuratively: "a sea of people invaded the main street", "the girl poured a sea of tears following the death of her little dog".