The conjunctions "that" and "which" are quite often used when writing
to connect to clauses. While many people use them interchangeably--I used to be one of those---, "that" and "which" convey a subtly different meaning. Consider the following two examples:
1. The books, which have green cover, are new.
2. The books that have green cover are new.
Can you see the difference between the two examples?