Alternatives to relatives 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.

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.

Example.- 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.