Defining and non-defining relative clauses

                         Defining relative clause

Relative clauses give us information about the person or thing mentioned. 

Defining relative clauses give us essential information – information that tells us who or what we are talking about.

The woman who lives next door works in a bank. 
These are the flights that have been cancelled.

We usually use a relative pronoun or adverb to start a defining relative clause: who, which, that, when, where or whose.

who/that

We can use who or that to talk about people. that is more common and a bit more informal.

She's the woman who cuts my hair.
He's the man that I met at the conference.

which/that 

We can use which or that to talk about things. that is more common and a bit more informal.

There was a one-year guarantee which came with the TV.
The laptop that I bought last week has started making a strange noise!

Other pronouns

when can refer to a time.

Summer is the season when I'm happiest.

where can refer to a place.

That's the stadium where Real Madrid play.

whose refers to the person that something belongs to.

He's a musician whose albums have sold millions. 

https://learnenglish.britishcouncil.org/grammar/intermediate-to-upper-intermediate/relative-clauses-defining-relative-clauses