Relative pronouns and relative adverbs introduce relative clauses.
'Who' - 'whose' - 'whom' - 'that' and 'which' - are relative pronouns.
'Where' is a relative adverb.

There is often confusion about the use of who, whose, whom, that, which or where.

• We use who when referring to people or when we want to know the person.
  • Who ate all the chocolates?
    
  • Who called the police?
    
  • The man who witnessed the accident called the police.
  • The person who answered the phone was very helpful.
    
    
• We use which to refer to a thing or an idea, and to ask about choices.
  • My car, which is 20 years old, isn't worth much.
  • The castle, which was built in the 18th century, is a local attraction.
  • Which size would you like: small, medium or large?
    
  • Which would you prefer: red or white?
    
    
• We use that for both a person and a thing/idea.
  It should be noted, however, that who is preferred when referring to a particular person.
  When that is used, it usually refers to classes or types of people.
  • This is the style that I want to use.
  • The car that belongs to the company is the red one over there.
  • She is the kind of person that/who can be trusted.
  • He's the sort of player that/who attracts the crowds.
  • I want to speak to the person who (not that) called me this morning.
  • The person who (not that) borrowed the dictionary should return it as soon as possible.
    
    
• Whose refers to ownership.
  • Whose dictionary is this?
  • I don't know whose coat this is.
  • There's the girl whose car was stolen.
  • The student whose article was published received a prize.
    
    
• Whom: 
  When who is the object of a verb, whom can be used instead, but it is formal and rather old-fashioned.
  In modern speech, we use who, or we leave out the pronoun.
  
  
  • To whom did you wish to speak? (formal)
  • Who would you like to speak to? (modern speech)
  • You are referring to a person who no longer works here.
  • The person to whom you are referring no longer works here.
  • The person (who) you are referring to no longer works here.
    
    
  Whom is always used when it is preceded by quantifiers such as all of, both of, few of, many of, several of, etc.
  For example:
  
  
  • She addressed the spectators, most of whom remained seated.
  • The mayor welcomed the boys and girls, all of whom wore their school uniform.
  • He greeted the soldiers, many of whom were injured.
    
    
• Where (relative adverb) refers to places and locations.
  • Where is the station please?
  • That's where I spent my childhood.
    
    

OMITTING RELATIVE PRONOUNS
When can we leave out relative pronouns (who, whom, which, that)?

In conversational English relative pronouns can be omitted when they are the object of a relative clause. In a formal context it is usually wiser to leave the relative pronoun.

Example:

Tom drives a red truck.

→ The person who drives a red truck is called Tom.
In this sentence 'who' refers to the subject so it cannot be omitted.

→ The truck (that) Tom drives is red.
In this sentence 'that' refers to the object (the truck) so it can be omitted.

RELATIVE PRONOUNS and RELATIVE CLAUSES

A relative pronoun is used to introduce a relative clause.
A relative clause is a description for a noun.
The description comes after the noun to identify it or give more information.

• A defining relative clause identifies a noun. It provides information necessary for identification.
  (These clauses are also called identifying relative clauses or restrictive relative clauses)
  Defining relative clauses are not  put in commas.
  
  The woman who is speaking is a friend of mine.
  
  The clause "who is speaking" clarifies which woman you are referring to.
  It is a defining relative clause.
  
  
• A non-defining relative clause adds information which is not essential for identification purposes.
  (These clauses are also called non-identifying relative clauses or non-restrictive relative clauses.)
  Non-defining relative clauses are put in commas.
  
  Ms. Smith, who is a friend of mine, is speaking about sustainable resources.
  
  The clause "who is a friend of mine" adds non-essential information.
  It is a non-defining relative clause.
  
  N.B. 'That' cannot replace 'who' to introduce a non-defining relative clause.
  
  You cannot say:
  Ms. Smith, that is a friend of mine, is speaking about sustainable resources.