Here in the United States, many teachers tell us to use "that" for defining clauses. That is, when the information is absolutely necessary: I want the book THAT has the red cover (not the blue cover). They tell us to use "which" for non-defining clauses. That is, information that may be helpful, but is not absolutely necessary. The commas show that the information can be omitted without any problem: Sacramento, which is the capital of my state, is very beautiful. IMPORTANT NOTE: For many English speakers outside the United States, they have NO problem in saying "I want the book WHICH has the red cover."

La differenza fra le due e' piu' o meno uguale a la differenza fra "che" e "cui," ma il modo in qui le usiamo e' un po' diverso. Paragoni: He is the man that I was talking about. Lui e' l'uomo di cui parlavo. I hope that everything is ok. Spero che tutto stia bene. That is the book in which Virgil travels with Dante. Quello e' il libro in cui Virgilio viagga con Dante. Ecco una spiegazione piu' complessa, ma forse ti possa aiutare: When referring to objects, though, the rule for using “that” and “which” correctly is simple: THAT should be used to introduce a restrictive clause. WHICH should be used to introduce a non-restrictive or parenthetical clause. A restrictive clause is one which is essential to the meaning of a sentence – if it’s removed, the meaning of the sentence will change. For example: Chairs that don’t have cushions are uncomfortable to sit on. Card games that involve betting money should not be played in school. To our knowledge, it is the only body in the solar system that currently sustains life… A non-restrictive clause can be left out without changing the meaning of a sentence. Non-restrictive clauses are either in brackets or have a comma before and after them (or only before them if they come at the end of a sentence): Chairs, which are found in many places of work, are often uncomfortable to sit on. I sat on an uncomfortable chair, which was in my office.