This question is very basic and frequently asked as for general knowledge. There is not such huge difference but a minor one is
Sea are smaller than the Oceans and are surrounded by land. Seas are land and ocean meet. Where as Oceans are larger body of water on the earth.
Sea are smaller part of ocean and are contained by land.
As summarizing the definition
Seas are the areas of water that are enclosed by land where as oceans are open water areas.