From the 4th Edition :
That. Which. That is the defining, or restrictive, pronoun, which is the nondefining or nonrestrictive. (See Rule 3.)
The lawn mower that is broken is in the garage. (Tells which one.)
The lawn mower, which is broken, is in the garage. (Adds a fact about the only mower in question.)
{{ But note however that, in practical terms, this comment is merely a style guide. Statistical analysis suggests that usage varies widely. }}