You use "which" to introduce a clause that's non-restrictive, i.e. you can take the clause out and not change the meaning of then sentence. You use "that" to introduce a clause that's restrictive, i.e. if you remove the clause, the meaning of the sentence changes. I think Grammar Girl explains this pretty well: http://www.quickanddirtytips.com/education/grammar/which-versus-that-0

Someone just asked the same question: https://www.italki.com/question/356008! Here's my answer I wrote at the time: I assume you mean in the role of relative pronouns, and that you are talking about things, not people or places. Short answer: you can always use "which", but you can sometimes substitute "that". "Which" can always be used in that role, whether as in "identifying" or "restrictive" relative clause, that is one that limits the noun it refers to what is described in the relative clause, or as a "supplementary" relative pronoun, which describes the noun without actually limiting. The second is distinguished by a comma that sets it off (as in the last sentence). Additionally, "that" can be used instead of which, but only as a restrictive relative pronoun, and only if it directly follows the noun that it describes (as I just used it). Some examples: - "The letter which/that I wrote" (object pronoun, restrictive) - "The letter which/that is in the mail" (subject pronoun, restrictive) - "The opera Don Giovanni, which I love" (object pronoun, supplementary) - "The job for which I applied" (object pronoun used with a preposition, restrictive) Additionally, "that" can also be used in a similar fashion as an alternative to "who" or "whom" in connection with people (although some people disapprove of this usage), whereas "which" really can't (although you may see some examples in older texts).

British and American style differ a bit. As an American, I use "that" or "who" whenever I can and the relative pronoun "which" only when necessary. (Use "who" instead of "that" or "which" when referring to a human being.) Examples: 1. The color that I like best is 2. The man who telephoned me yesterday 3. The thing that is hardest to understand is Note that the word "that" can often (but not always) be omitted: 4. The color I like best is So when should you use "which"? You use it when the information in the which-clause is extra or optional, rather than essential to the meaning of the sentence. Usually, in addition to using "which," you also use commas or parentheses in such cases: 5. The color, which I had never seen before, was a bright blue 6. The explanation (which is not easy to understand) is that You cannot use "that" in sentences 5 and 6. The word "which" can also be an interrogative pronoun: 7. Which color do you like best? As an interrogative pronoun, "which" cannot be replaced by "that." Notice the difference between the following two sentences: 8. The cats that live in the basement ... 9. The cats, which live in the basement, ... In 8 you are talking specifically about the cats that live in the basement. In 9 you are talking about "the cats" and adding as additional, optional information that they happen to live in the basement.

"I am only 17 years old, which means I am not old enough to vote" "I am only 17 years old, that means I am not old enough to vote" I think the two words (thats and which) are interchangeable, there may be a case where they are not but Im not sure at the moment.