What is the difference between a sea and an ocean?

The basic distinction is size and enclosure. Oceans are the enormous, deep, open bodies of saltwater that cover most of the planet, and there are five of them: the Pacific, Atlantic, Indian, Southern (around Antarctica), and Arctic. Seas are smaller bodies of saltwater, usually partially enclosed by land, and they are typically located where the ocean meets the land or are branches and marginal parts of an ocean. The Mediterranean Sea, the Caribbean Sea, the Red Sea, and the South China Sea are all examples, smaller, more enclosed by surrounding coasts, and generally shallower than the open oceans they connect to. So in the loose everyday sense: oceans are the vast, open, deep expanses, and seas are the smaller, more land-embraced pockets, often part of or bordering an ocean.

The terminology gets fuzzy at the edges, because the words are used loosely and some names defy the pattern. A few bodies called seas are actually landlocked saltwater lakes with no connection to an ocean, the Caspian Sea and the Dead Sea are technically enormous salt lakes, not seas in the ocean-connected sense, and they keep the sea label by tradition. People also use sea casually to mean the ocean in general (going out to sea, seaside), blurring the distinction further. And the boundaries between one ocean and another, or where a sea ends and the open ocean begins, are human conventions drawn on maps rather than sharp physical lines, since all the world's oceans and seas connect into one continuous global body of saltwater. So the clean rule of thumb, oceans are the big open ones (five of them), seas are the smaller, more enclosed ones (dozens), often part of an ocean, works for most cases, with a handful of traditionally named exceptions that are really salt lakes.