Which vs. that

“Of all the gin joints in all the towns in all the world, she walks into mine.”
—Rick Blaine (Humphrey Bogart) from the 1942 movie, Casablanca

When Rick says this line in Casablanca, he is narrowing down, or restricting, his discussion from many possible gin joints down to the one he owns. In other words, of all the gin joints it could have been, she walked into that particular one. This concept of mentioning an item of which there are many, then adding more information
to narrow it down to the specific one you mean, is called “restricting the discussion.” From this concept comes the grammatical word for this: restrictive.
Why do we need to understand the concept of “restrictive”? Because it helps us know when to use “which” and when to use “that” when we are adding additional description to a sentence.

The words "that" and "which" are both used to add further description after a noun in a sentence. The word "that" is used when the additional information is needed in order to restrict the discussion down to the exact noun that is meant. "Which" is used to add extra information that is not really needed in order to identify the exact noun.

Let’s look at a specific example.

A. “The river is very muddy today.”
B. “The river that flows from Minnesota to Louisiana is very muddy today.”

As a reader of sentence A, you are probably wondering, “what river?” The noun “river” could mean any river in the world. You need more information to help you identify exactly what river I am talking about. When I add “that flows from Minnesota to Louisiana” in sentence B, you probably now realize exactly what river I mean: the Mississippi. The clause “that flows from Minnesota to Louisiana” narrows my discussion down, or restricts it, from all the possible rivers in all the world to the one I am talking about: the Mississippi. The word that is used to begin what is called a restrictive clause.

Here are two different examples:

C. The Atlantic Ocean separates England from the United States.
D. The Atlantic Ocean, which is sometimes called The Big Pond, separates England from the United States.

The word "which" adds information that is not needed to restrict the discussion. When I start sentence C with “The Atlantic Ocean,” I have already specified the exact ocean. You do not need any more information to help you identify exactly which gin joint—I mean ocean—I am referring to. In other words, no further restriction is needed to make my meaning clear. But I just felt like adding further description or information, so I added “which is sometimes called The Big Pond” in sentence D. I put this additional information inside two commas to indicate that it is not really necessary.

You can think of the two commas around a nonrestrictive clause as miniature parentheses, indicating that the clause is merely parenthetical. You could take out the part of the sentence between the commas, and the reader would still understand the sentence and not be left wondering what you were talking about.

Use this little chart as a reminder:

That is restrictive (and essential)and has no commas.
Which is nonrestrictive (and nonessential) and has commas.