Kelly asked Lucy Merrill:

What's the difference between "which" and "that" and when should I use which (or should I say that)? :)

Lucy Merrill "That" introduces a restrictive clause and "which" introduces a non-restrictive clause. That's just the English class way of saying it (so you'll be confused forever). Plain English: If you can take out the clause and your sentence still makes sense and conveys the basic information you intended, use "which." If the clause absolutely has to be there, use "that."
The dog that bit the mailman had to be quarantined.
The dog, which bit the mailman last week, roamed the neighborhood every day.

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