Who, that, and which are relative pronouns. We use relative pronouns to introduce relative clauses. Relative clauses tell us more about people and things.
Examples
John Smith, who is 68, has just retired.
You should take the job that pays the most.
My tuxedo, which I bought last week, fits me perfectly.
Relative pronouns are used in so-called relative clauses to provides information about another word in the sentence (usually the word immediately to the left).
Use who when you refer to a person or people.
Examples
The runner who won the race is a cancer survivor.
The person who answered the phone was very helpful.
My sister, who lives in Prague, is coming home tomorrow.
Use that for things and, informally, for people. However, in a formal text, always use who when you refer to people. If you don’t use a comma to describe an object, use that. If you use a comma, use which.
Examples
The dress that you bought is beautiful.
I want a car that accelerates quickly.
You should take the job that pays the most.
The boy that stole your bike is at the door. (informal)
Use which when you refer to a thing. If you use a comma to describe an object, use which. If you don’t use a comma, use that.
Examples
Sally’s jacket, which she bought at a market, is too small.
My wedding dress, which I bought last week, fits me perfectly.
My Mac computer, which I bought in Stockholm, is amazing.