What are the differences in the usages of the relative pronouns 'that' and 'which' in sentences?

Question

What are the differences in the usages of the relative pronouns 'that' and 'which' in sentences?

Answer 1

Mike Y is spot on. 'Which' is used for clauses that are not essential to the sentence, thus, are set off by commas. Whereas, 'that' is used for clauses that are essential elements of a sentence; that is, if you take out that part of the sentence it does not say what you wanted to say.

Answer 2

That = used for clauses that are essential to understanding the meaning of the sentence.

Which = used for clauses that are not essential to understanding the meaning of the sentence.

Ex:

The book that I love is on the desk.  (In other words, I only love one book, the one on the desk).

The book, which I love, is on the desk.  (The book is on the desk.  I happen to love it, but I could also love many other books.)

Basically, if you can take the clause out of the sentence and you're still saying what you mean to say, then the clause is non-essential and should be preceded by 'which' and set off by commas.  Otherwise, use 'that'.