Relative clauses are the part of the sentence that describes the object. For example, in the sentence. For example: Do you see that man in the car? The part of the sentence, ...in the car, is the relative clause. There are two kinds of relative clauses, Defining, and Non-defining:

1) Defining relative clause: Defines the object; in this kind of relative clause the definition of the object is the important part of the sentence.

         Example: I like people who are fun to be with.

The sentence above has a defining relative clause because the reason for the sentence is to say that people, who are fun to be with, are the kind of people I like.

2) Non-defining relative clause: doesn't define the object, so
    the clause is not as important, it is just put in there as an afterthought.

         Example: James, who is in the other room, likes that movie.

The sentence above has a non-defining relative clause because the reason for the sentence is to express that James likes that movie. The fact that he is in the other room is just there as extra information.

* Non-defining relative clauses are usually used in writing, where defining relative clauses are usually used in conversation.

         Practice - Are these sentences Defining (D), or non-defining (ND) relative clauses?

1) The key is the thing you use to unlock doors. D ND
2) My friend Kyle, who is from Hawaii, likes to surf. D ND
3) Politicians who tell lies are odious. D ND
4) Politicians, who tell lies, are odious. D ND
5) My favorite car is the Toyota Camry, which will be out of production next year. D ND
6) His latest book, which got good reviews, has sold a lot of copies D ND
7) I met a guy who works at the ski resort. D ND
8) The main way I learned English was by memorizing words. D ND
9) James, who has been skiing since he was five, is a very good skier. D ND
10) My son, who is going to Africa next year, likes to travel. D ND
11) You look sick, I'll get you my son who is a doctor. D ND
12) I'll loan you that book that changed my life. D ND