Some relative clauses identify nouns. They tell us which noun or pronoun is meant. These kinds of clauses are called identifying, defining or restrictive relative clauses.
The man who stole my car has been arrested by the police. (Here the relative clause ‘who stole my car’ identifies the noun man.)
The book which you see on the table cost me twenty dollars. (Here the relative clause ‘which you see on the table’ identifies the noun book.)
People who take physical exercise lead healthy lives.
Other relative clauses do not identify or classify nouns.They merely give additional information about a person or thing that is already identified. These kinds of relative clauses are called non-identifying, non-defining or non-restrictive relative clauses.
My brother, who is a doctor, lives abroad. (Here the relative clause who is a doctor does not identify the noun my brother. It simply gives additional information.)
Identifying relative clauses usually come immediately after the nouns that they modify. In speech, they are not separated by pauses.
Non-identifying relative clauses are normally separated by pauses in speech and commas in writing.
The boys who stood first in the examination were given prizes.
John, who stood first in the examination, was given a prize.
Note that non-identifying relative clauses can be easily left out.
John was given a prize.
Identifying relative clauses cannot easily be left out.
People lead healthy lives. (Which people? This sentence does not mean the same as ‘People who take physical exercise lead healthy lives.)
The relative pronoun that is common in identifying relative clauses. That is unusual in non-identifying relative clauses.