which, who and that - the difference

The Quick Answer
Use which for things and who for people. Use that for things and, informally, for people.

When do you need a comma before which and who?

If the who or which clause is just additional information (i.e., you would be happy to put it in brackets), then you should offset it with commas.

Which, That and Who

The words which, who, and that are grammar villains — they are often the cause of grammar errors. Most commonly, this stems from confusion over whether to use a comma before which or who. Unfortunately, the rules are not simple. They are explained in more detail in the following lessons:

Which, who and that.
(A lesson providing an overview of which, that, and who)

Commas before which and who.
(A lesson focusing on when to use commas with which and who)

No commas before which and who.
(A lesson focusing on when not to use commas with which and who)

These lessons are all quite similar, but they approach the issue from slightly different perspectives.
Interactive Test

See Also

adverse or averse? affect or effect? appraise or apprise? avenge or revenge? bare or bear? complement or compliment? dependant or dependent? discreet or discrete? disinterested or uninterested? e.g. or i.e.? envy or jealousy? imply or infer? its or it's? material or materiel? poisonous or venomous? practice or practise? principal or principle? tenant or tenet? who's or whose?
Commas before which and who No commas before which and who List of easily confused words