That. Which.

From Strunk and White:

That is the defining, or restrictive pronoun, which the nondefining, or nonrestrictive.

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)

I always forget these little rules, but they are worth remembering in all forms of writing.