Some relative clauses 'identify' nouns. They tell us which person or thing is meant.

• What's the name of the tall man who just came in?
  
• Whose is the car that's parked outside?
  (that's parked outside tells the hearer which car is meant: it identifies the car)

Other relative clauses do not identify. They tell us more about a person or thing that is already identified.

• This is Ms Rogers, whom you met last year
  (whom you met last year does not tell us which woman is meant: we already know that it is Ms Rogers.)
• Have you seen my new car, which I bought last week? (which I bought last week does not tell us which car is meant: we already know that it is 'my new car'.)

Non-identifying clauses are separated from the rest of the sentence bycommas (, ,). Identifying clauses do not have commas. Compare:

• The woman who does my hair has moved to another hairdresser's.
• Dorothy, who does my hair, has moved to another hairdresser's.

We only use that in identifying clauses. And we can only leave out the object in identifying clauses. Compare:

• The whisky (that) you drank last night cost £15 a bottle.
  
• I gave him a large glass of whisky, which he drank at once.
  (NOT ... whisky, that he drank ...) (NOT .. . whisky, he drank . . .)
  

Whom is unusual in identifying clauses. Compare:

• The man (that) my daughter wants to marry has been divorced twice.
• Max Harrison, whom my daughter wants to marry, has been divorced twice.

Non-identifying clauses are unusual in an informal style.