Seas are found on the margins of the ocean and are partially enclosed by land. Here, you can see that the Bering Sea is part of the Pacific Ocean, while the Chuckchi and Beaufort seas form part of the Arctic Ocean.
Many people use the terms "ocean" and "sea" interchangeably when speaking about the ocean, but there is a difference between the two terms when speaking of geography (the study of the Earth's surface).
Seas are smaller than oceans and are usually located where the land and ocean meet. Typically, seas are partially enclosed by land.