List of relative pronouns:

who and whom refer to people:

The man who is sitting at the head of the table is the  manager of a large car company.

  whom is used when the relative pronoun goes after a preposition:
Jeremy does not like June, with whom he has worked for two years.
And, sometimes, in formal English when it is the object of a verb:
Mrs Johnson loves Mary, whom she adopted two years ago.

  which refers to things: The car which you bought last year is very fast.

  that can be used instead of who or which in defining relative clauses:
The novel that / which I am reading is very good. 
The man that / who phoned this morning didn´t say what he wanted.

whose is used to show who or what something belongs to or is related to. It is not a pronoun because it always goes before a noun, so it is a kind of possessive determiner. But for the sake of clarity it is usually included in the list of relative pronouns:
Jane tried to console the woman whose husband died while he was helping rescue two mountaineers.

A relative pronoun can be:

  the subject of a relative clause: The man who phoned a moment ago wanted to speak to Susan.

  the object of a relative clause: The second-hand car which Paul bought last year  has not broken down yet.

  The relative pronoun can be omitted when it is the object of a defining relative clause:
The second hand car (that / which) Paul bought last year has not broken down yet.

  the complement of a preposition: The boy with whom Jenny goes out is a very good sportsman.

  The preposition can be put at the end of the relative clause and then you can use who instead of whom. If there is a time adverb / time expression, the preposition is placed before the time adverb: June, who Peter has gone out with for three months, is very nice.

• If it is a defining relative clause, you can omit the relative pronoun:
  The boy who Jenny goes out with is a very good sportsman.
  The boy Jenny goes out with is a very good sportsman.