People often confuse "which" and "that" when they are used as the introductory word in an adjective clause. They have become generally interchangeable in everyday speech, but the rules of grammar are pretty clear and it's good to know the correct usage. Fortunately, the built-in grammar check feature of most word processors is pretty good at catching this error and helping you fix it.
THAT is used to introduce "essential" information: information that's required to understand the identity of the thing it's describing. It doesn't get set off with a comma.
Example: He used the bat that he bought at the Baseball Hall of Fame. (The implication is that he has more than one bat, so the underlined clause is required.)
WHICH is used to introduce "non-essential" information: extra information that isn't required to understand the identity of the thing it's describing. It gets set off with a comma.
He used his bat, which he bought at the Baseball Hall of Fame. (The implication here is that he only has this one bat, so the fact that he bought it at the Baseball Hall of Fame is extra information, but not required to identify the bat.)
* Obviously, there are other uses of "which" and "that," but they aren't the focus of this post.