Someone told me that the sea & the ocean are actually different things. Is this true ?

Answer 1

Seas lie on top of continental crust. Oceans lie on top of oceanic crust. And that is the difference.

Answer 2

Yes, there's a difference.
Generally, there are two ways of defining both sea and ocean, geological and geographical.
I will give you the definitions I have been taught. Unfortunately, by browsing books and the Internet (and listening to different teachers), you will find a confusing array of different definitions, some innacurate and outdated.

Geology:
A sea is a mass of water above continental crust.
An ocean is a mass of water above oceanic crust.

Geography:
A sea is a mass of salt water completely or partially surrounded by land.
There are mediterranean seas (between continents), marginal seas (along the margins of continents), inland seas, and closed (completely cut off from other, large bodies of water). Most numerous are marginal seas.
An ocean is a huge mass of salt water covering 71% of the Earth. It is known as the World Ocean, although, traditionally, it's being divided into Atlantic, Pacific, Indian, Southern and Arctic Ocean (although some geographers refuse to separate Arctic from Atlantic Ocean).

Answer 3

When somebody refers to "the sea" they are usually referring to the earth's oceans. There are some seas that are landlocked, but for the most part, seas are just isolated parts of the ocean. So they can be different (as in the Caspian Sea or the Aral Sea) - but usually they are one and the same.

Answer 4

there are seven seas and four oceans. Like the Red sea and the Indian Ocean

Answer 5

Ocean surrounds the land, land surround the sea.

Answer 6

Size matters.

Answer 7

yes. a sea is smaller.