Who vs. That vs. Which

Rule 1

Who refers to people. That and which refer to groups or things.

Examples:
Anya is the one who rescued the bird.
Lokua is on the team that won first place.
She belongs to an organization that specializes in saving endangered species.

Rule 2

That introduces essential clauses while which introduces nonessential clauses.

Examples:
I do not trust products that claim "all natural ingredients" because this phrase can mean almost anything.
We would not know which products were being discussed without the that clause.
The product claiming "all natural ingredients," which appeared in the Sunday newspaper, is on sale.
The product is already identified. Therefore, which begins a nonessential clause.

NOTE: Essential clauses do not have commas surrounding them while nonessential clauses are surrounded by commas.

Rule 3

If this, that, these, or those has already introduced an essential clause, you may use which to introduce the next clause, whether it is essential or nonessential.

Examples:
That is a decision which you must live with for the rest of your life.
Those ideas, which we've discussed thoroughly enough, do not need to be addressed again.

NOTE: Often, you can streamline your sentence by leaving out which.

Example:
That is a decision which you must live with for the rest of your life.

Better:
That is a decision you must live with for the rest of your life.