What is a relative clause?

We use relative clauses to give extra information about something. We can get more information into a sentence without the need to start a new one.
 There are two types of relative clauses: defining and non-defining:
'Saris which are made of silk are expensive.' = defining relative clause
'Saris, which are made of silk, are expensive.' = non-defining relative clause

• The first sentence tells us that silk saris are expensive (we can guess there are other saris made of cheaper materials). This is a defining relative clause.>

• The second sentence tells us that saris are expensive and that they are ALL made of silk. The information between the two commas ",which are made of silk," is extra information and can be taken out and the sentence meaning remains the same 'Saris are expensive'. This is a non-defining relative clause. This type of clause is common in written English. In spoken English a pause is used instead of a comma.

Relative Pronouns

In both defining and non-defining relative clauses we us the following relative pronouns:
Who = is for people: 'The person who lives next door is a doctor.'
Whose = is used to show ownership/possession 'The woman whose son you met.'
Whom = object pronoun, although 'who' can be used in informal English. 'I saw the man whom you saw at the party.'
Which = is used for things: 'The car which you bought is cool.'
That = Can be used informally instead of ‘who’ and ‘which’.

Relative Adverbs

Relative adverbs are used like relative pronouns:
When = is used for a time. 'The time when I last saw you.'
Where = is used for a place 'The place where I used to live.'

Why = is used for a reason 'The reason why I am late.'

Reducing Relative Clauses

If the pronoun ("that", "who", "which") is the object of the verb, it can be omitted.
pIn defining relative clauses, when the pronouns 'that', 'who' and 'which' are the objects of the verb they can be taken out and the meaning of the sentence stays the same. Look:

'The man (that) I work with collects snakes.'
'The person (who) I spoke to knows you.'
'The shop (which) she likes has closed down.'

When the pronoun is the subject of the sentence it must be used:

'The shop which hired her has closed down.'

Sumber :
http://www.ecenglish.com/learnenglish/what-a-realtive-clause/