Relative clauses give more information about a subject or object. They usually follow and "agree" with the noun they modify and often occur between a Subject and Verb. However, they usually have no effect on the S + V relationship.
The man who works at IBM comes from Hong Kong.
The house that Jack built remains empty.
The people who came to the party had a great time.
Those who arrive early are entitled to a rebate.
I ate an apple that had a worm in it.
She is the one who I told you about.
The man who lives over there is my uncle.
One of the men who lives over there is my uncle.
Only one of the people who work in the company is qualified.
Most relative clauses use the words who, whom, whose, which, that, when or where.
This is the place where I met my wife.
Paul is the man who loves Mary.
Simon is the man who(m) Mary loves.
Commas which set off relative clauses function like parentheses ( ) indicating non-essential information.
My wife, who is a doctor, works at Community Hospital.
My wife (who is a doctor) works at Community Hospital.
Without commas, relative clauses specify one member of a group:
My brother who is a scientist works at the university.
My brother who is a mechanic works at Bob's Garage.
Specifies "which brother" (one of many)