GRammar FOcus:

Defining Relative Clauses 

Defining relative clauses give detailed information defining a general term or expression. Defining relative clauses are not put in commas.

Example:
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.

Non-Defining Relative Clauses 

Non-defining relative clauses give additional information on something, but do not define it. Non-defining relative clauses are put in commas.

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