Look at this example sentence:

• The woman who lives next door ... ('who lives next door' tells us which woman)
• People who live in the country ... ('who live in the country' tells us what kind of people)

• An architect is someone who designs buildings.
• What was the name of the person who phoned?
• Anyone who wants to apply for the job must do so by Friday.

• The woman that lives next door is a doctor. (not the woman which)

Sometimes you must use who (not that) for people - see Unit 95.

When we are talking about things, we use that or which (not who) in a relative clause:

• I don't like stories that have unhappy endings. (or stories which have ... )
• Grace works for a company that makes furniture. (or a company which makes furniture)
• The machine that broke down is working again now. (or The machine which broke down)

That is more usual than which, but sometimes you must use which - see Unit 95.

• 'Who's that woman ?'  'She lives next door to me.'
  I've never spoken to the woman who lives next door. (not the woman she lives)
• Where is the cheese? It was in the fridge.
  Where is the cheese that was in the fridge? (not the cheese it was)