Use which for things and who for people. Use that for things and, informally, for people.|
Sometimes, you need a comma before which and who. The rule is this: If the information added by the who or which clause is just additional information (i.e., it's not essential to identify another word in the sentence), then you should offset the clause with commas.
Using Which, Who and ThatThis page is about the relative pronouns which, who, and that. They are used to link information (in the form of a phrase or a clause) to another word in the sentence.
Who is used for people. Which is used for things, and that can be used for either. It is quite unfashionable to use that for people. (The consensus seems to be that using that for people is still acceptable in speech and informal writing, but you should avoid doing it in formal writing.)
Commas with Which and WhoThe biggest issue for native English speakers is when to use a comma before which and who. Unfortunately, the ruling is not simple. It is covered in the following lessons:
WHAT ARE RELATIVE PRONOUNS?
Which and who are relative pronouns. There are others, but these two are the most common. (See lesson Pronouns for more information.)
The other one covered in this section is that. All three (which, who, and that are used to link to another word in the sentence (very often the one directly to the left) to add information about that word.
WHEN DO YOU PUT A COMMA BEFORE WHICH AND WHO?
This is a nasty subject. When who or which introduces a restrictive phrase or clause (i.e., additional information that is required to identify the word it is linked to), then the who or the which can be replaced by that. If the who or which introduces information that is not essential to identify the word it is linked to (called the antecedent), then the who or the which will be offset with commas and it cannot be replaced with that.
This infographic might make it easier to understand:
There is more on whether commas are required with who or the which and whether they can be replaced with that in this lesson:
NOT AT THE START OF A SENTENCE
Do not start a sentence with words like which and who (unless it is a question).
should be ...extra room, which gives...
WHOSE AND WHO'S
Who's is a contraction of either who is or who has. It has no other uses.
Whose, on the other hand, usually sits before a noun to introduce information or ask a question relating to ownership.
(Whose in this example is an interrogative pronoun. It is asking a question about the ownership of the bike.)
This is covered more in the lesson Who's and Whose.