👍Defining Relative Clauses
Defining relative clauses (also called identifying relative clauses or restrictive relative clauses) give detailed information defining a general term or expression. Defining relative clauses are not put in commas.

Imagine, Tom is in a room with five girls. One girl is talking to Tom and you ask somebody whether he knows this girl. Here the relative clause defines which of the five girls you mean.

Do you know the girl who is talking to Tom?

Defining relative clauses are often used in definitions.

A seaman is someone who works on a ship.

Object pronouns in defining relative clauses can be dropped. (Sentences with a relative clause without the relative pronoun are called Contact Clauses.)

The boy (who/whom) we met yesterday is very nice.

👍Non-Defining Relative Clauses
Non-defining relative clauses (also called non-identifying relative clauses or non-restrictive relative clauses) give additional information on something, but do not define it. Non-defining relative clauses are put in commas.

Imagine, Tom is in a room with only one girl. The two are talking to each other and you ask somebody whether he knows this girl. Here the relative clause is non-defining because in this situation it is obvious which girl you mean.

Do you know the girl, who is talking to Tom?

💥💥Note: In non-defining relative clauses, who/which may NOT be replaced with that.💥💥

Object pronouns in non-defining relative clauses must be used.

Jim, who/whom we met yesterday, is very nice.