Grammar Girl : Which Versus That :: Quick and Dirty Tips ™:
"which" is for non-essential or additional information.
"that" is a required limitation or clarification.
Diamonds that are expensive make great gifts.
This says that expensive diamonds are great gifts.
This must stay as one sentence to make its point.
Diamonds, which are expensive, make great gifts.
This says that diamonds (all diamonds) are expensive and that all diamonds make great gifts.
This could be split into two sentences and still have the same meaning.
