Subject Relative Clauses

Who, that, and which are subject relative pronouns which are used to join two English sentences, or to give more information about something.  A relative clause defines or gives information about the noun that it follows.

Use that to talk about people or things.
Use who and whom to talk about people, and which to talk about things.

Examples: 
She bought a blouse. The blouse is red.
She bought a blouse that is red.

Defining Relative Clause - tells which noun we are referring to, this information is necessary to understand the sentence.

I was working at the company that went bankrupt.
(If we don't say that went bankrupt we don't know why I don't work there anymore)

Non-defining Relative Clause - gives extra information about something, but it is not necessary to understand the sentence.

Marie, who was married to Steve, is coming to Madrid this week.
(What is important is that she is coming to Madrid this week, that she was married to Steve is extra information.)

Defining And Non Defining Relative Clauses from South Thames

More Information on Relative Clauses
What is a Relative Clause?

Relative Clauses Powerpoint