A clause is a grammatical unit containing a subject and a
predicate. Every sentence, therefore, contains at least one
clause, but it may contain more.
A relative clause relates one clause to a particular word in
another clause. A relative clause normally begins with a
relative pronoun: who or whom, which, or that.
Relative clauses can be either restrictive or nonrestrictive.
A restrictive clause restricts the reference of the term it
modifies (a clause).
The Japanese who eat lots of fish have fewer heart attacks.
The subordinate clause restricts the reference of the term
"Japanese" to a certain subclass of the Japanese people: those
who eat lots of fish. As a result, we are making a single
statement about that subclass, and we are not making any
statement about the Japanese people as a whole.
A nonrestrictive clause doesn't restrict that term's reference.
The Japanese, who eat lots of fish, have fewer heart attacks.
This proposition asserts that the Japanese have fewer heart attacks
and that they eat lots of fish. It makes two statements
about the Japanese people as a whole.