via Tips for Writers
That vs. Which (from http://www.dailywritingtips.com)
Just a reminder, who should always be used when referring to people.
The boy who threw the ball.
This is the woman who won the pie-eating contest.
When referring to objects, the rule for using that and which is simple.
That should be used to introduce a restrictive clause.
Which should be used to introduce a non-restrictive or parenthetical clause.
A restrictive clause is one which is essential to the meaning of the sentence – if it is removed, the meaning of the sentence with change.
Examples: Chairs that don’t have cushions are uncomfortable to sit on.
Card games that involve betting money should not be played in school.
A non-restrictive clause can be left out without changing the meaning of the sentence. Non-restrictive clauses are either in brackets or have a comma before and after them.
Examples: Chairs, which are found in many places or work, are often uncomfortable to sit on.
I sat on an uncomfortable chair, which was in my office.
Changing that to which or vice versa can completely change the meaning of a sentence.
My car that is blue goes very fast. (It is the blue car, not the green one or the white one that is fast.)
My car, which is blue, goes very fast. (My car is fast, and the color is blue.)
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