Technically, there is a difference between an ocean and a sea. An ocean is an extremely large body of salt water, and a sea is partially surrounded by land.
There are only five oceans: the Atlantic Ocean, Pacific Ocean, Indian Ocean, Arctic Ocean, and Antarctic Ocean. There are more than 50 seas, including the Mediterranean Sea, the Baltic Sea, Caribbean Sea, and South China Sea.
Most seas are connected to the ocean, although there are a few that are completely surrounded by land, like the Aral Sea, Caspian Sea, and Dead Sea.
However, when talking about salt water in general, many people use ocean and sea interchangeably.
A lake is an area of fresh water (not salty) that is completely surrounded by land. A pond is the same as a lake, only smaller.