Relative Clause

RELATIVE CLAUSE
We use relative clause to give additional information about something without starting another sentence. By combining sentences with a relative clause, your text becomes more fluent and you can avoid repeating certain words.

How to form relative clause?

Imagine, a girl is talking to Tom. You want to know she is and ask a friend whether he knows her. You could say:

"A girl is talking to Tom. Do you know the girl?"

That sounds rather complicated, doesn't it? it would be easier with a relative clause: you put both pieces of information into one sentece. Start with the most important thing: you want to know who the girl is.

"Do you know the girl....."

As your friend cannot know which girl you are talking about, you need to put in the additional information - the girl is talking to Tom. Use "the girl" only in the first part of the sentence, in the second part replace it with the relative pronoun (for people, use the relative pronoun "who"). So, the final sentence is:

"Do you know the girl who is talking to Tom?"

Relative Pronoun
Who : subject or object pronoun for people
Which : subject or object pronoun for animals and things
Whose : possession for people, animals, and things
Whom : object pronoun for people, especially in non-defining relative clauses (in defining relative clauses we colloquially prefer who)
That : subject or object pronoun for people, animals, and thing in defining relative clauses

(Source:  http://www.ego4u.com/en/cram-up/grammar/relative-clauses )