A restrictive clause is also called a defining clause, and a nonrestrictive clause is called a non-defining clause. A restrictive or nonrestrictive clause usually appears in the middle of a sentence. There is one big difference between these two clauses. A restrictive clause adds essential information and its removal will affect the meaning of the sentence. A sentence that has a restrictive clause does not have any commas. A nonrestrictive clause contains additional information, which is not essential to what the sentence is saying, and is separated from the sentence by commas – a comma before and a comma after it.
(1) My uncle who is a black belt at karate was beaten up by ten men.
The clause who is a black belt at karate identifies the uncle who is being talked about. Without this clause, it is just possible you do not know who the uncle is as I may have more than one uncle. No commas come before and after this clause, which is a restrictive/defining clause.
(2) Bobby, who is a black belt at karate, was beaten up by ten men.
The clause who is a black belt at karate in the second example is a nonrestrictive clause. Although it provides extra information about Bobby, it is non-essential information because we already know who Bobby is. Commas are used to separate this clause, which if removed from the sentence does not affect the sentence.