Defining: if we remove the relative clause the sentence doesn't usually make sense.

• We don't use any commas

      This is the jumper I've bought. 

• We can omit the relative pronoun if it's not the subject:

      I have eaten all the biscuits (that) you bought. 

• When, where and why can be omitted:

      I remember the day (when) we went skiing. 

Non-defining: it gives us more information about someone or something. If we remove the relative clause it still makes sense.

• We cannot use that, why or what.
• We must use a comma or commas.

         Mrs Evans, who is a pianist, is coming tonight. 

• We cannot omit where and when. 

      Farnborough, where I was born, is in       southeast England. 

• Who- people or animals (to give them a personality)
• Which- things or animals 
• When- to refer to time
• Where- to refer to a place or situation
• Why- to give a reason
• Whose- the possessive of who and which
• What- the thing(s) which. Only in defining clauses. 

FORMAL.

• Whom- formal for who (basically used in writing)

INFORMAL

• That- who, which, when, where, why. Only in defining  clauses. 

• There are a lot of people who don't like bitter things.
• This is the cardigan which I got for Christmas.
• Can you remember the days when you had to walk to school? 
• Alaska is a place where it never gets dark from May to July. 
• And that's the reason why I wanted to go in bed early.
• There are children in the school whose parents are from abroad.
• I'll give you what I promised.

• That's the lawyer with whom you have to talk. 

• Charlie is the neighbour that I see the least.