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'.)