Relative Pronouns

Usually, a relative pronoun, like other pronouns, qualifies or modifies a noun or noun clause. It links a phrase or a clause to a noun.

Important relative pronouns are:

Who------humans----replaces he, she, it, they 

It was Gandhiji who roused the nation from its slumber.

Whose----shows possession or ownership

This is the girl whose photo was published in the newspaper.

Whom----used in objective case----replaces me, her, him

The guests whom the president invited are here.

Which-----objects and animals----are usually non defining in nature

The mail, which I usually get in the afternoon, is junk.

That-----humans, objects, animals----are usually defining in nature

This is the movie that everyone is raving about.

Defining/ Restrictive clause

This clause gives vital information about the noun in consideration. For a sentence to retain its meaning it cannot be deleted. It requires no additional punctuation. 

I don't like people who do back-biting.

Non-Defining/non-restrictive clause

Non Defining clauses give superfluous information that's nice to have but isn't crucial to the overall meaning of the sentence. Even if they are deleted, the sentence would convey essentially the same meaning. Therefore, non-defining clauses are written using commas, which help to indicate that they are redundant or less important in the sentence.

The doctors are trying a new technique, which will revolutionize the world.