Lio - that vs. which. Essentially there are 2 kinds of relative clauses - restrictive and non-restrictive (parenthetic). Restrictive clauses are so-called because they restrict the nouns they modify, giving them some necessary information, e.g.
"The meat that I bought from the butcher was rancid."
Which can also be used in restrictive clauses, e.g.
"The meat which we eventually served was rancid."
Note, these clauses tend to drop the "that" in spoken language. If you can drop the "that" then it's probably a restrictive clause.
Non-restrictive clauses: It does not give necessary information, e.g.
"The meat, which was rancid, was nonetheless surprisingly tasty."
"I charged double for the meat, which I gave to the Scouts."
If you use "that" in the above sentences, you change the meaning, e.g.
"The meat that was rancid was nonetheless tasty."
(Implies there is other meat that isn't rancid)
"I charged double for the meat that I gave to the Scouts."
(Implies there is more meat that wasn't given to the Scouts).