Defining relative clauses are used to specify which person or thing we mean. We don't put commas between the noun and a defining relative clause.
Who or that are used for people.
Which or that are used for things.

I have a friend who speaks five languages.
I have a friend that speaks five languages.

She showed me the coat which she had bought.
She showed me the coat that she had bought.

Non-defining relative clauses (extra information clauses) are used to add extra information which is not necessary. We put commas between the noun and a non-defining relative clause.
Who is used for people.
Which is used for things.
That cannot be used.

Mr Fry, who speaks five languages, works as a translator for the EU.

The area, which has very high unemployment, is in the north of the country.

The man who was sitting next to me had a brown jacket.

After a relative clause we do not repeat the subject.

The book that I bought was by Edward Lear.

Only that or which are possible, not what.

He lent me the book, which I found very useful.

A relative clause can have only one direct object.

The winner, who was 25, will receive $12,000.

That cannot be used in a non-defining (extra information) clause.

Our office, which I share with my two collegues, is about two kilometres from the city centre.

A relative clause follows the noun to which it refers.